🔤 Lesson 10: Strings & String Functions

String Basics Review

A quick refresher on what we covered in Lesson 3 about strings, plus a few new details.

Single vs. Double Quotes

<?php
$name = "Alice";

// Double quotes — variables ARE interpolated
echo "Hello, $name!\n";        // Hello, Alice!
echo "Tab:\there\n";            // Tab:    here  (escape sequences work)

// Single quotes — variables are NOT interpolated
echo 'Hello, $name!\n';        // Hello, $name!\n  (literal text)
echo 'Only two escapes: \' and \\';  // Apostrophe and backslash

// Complex interpolation with curly braces
$user = ["name" => "Alice", "role" => "admin"];
echo "Welcome, {$user['name']}! Role: {$user['role']}\n";
echo "She has {$count} item" . ($count !== 1 ? "s" : "") . "\n";

String Concatenation

<?php
// Dot operator (.) joins strings
$first = "Hello";
$second = "World";
$greeting = $first . ", " . $second . "!";
echo $greeting; // "Hello, World!"

// Concatenation assignment (.=)
$message = "Step 1: Open the file. ";
$message .= "Step 2: Read the data. ";
$message .= "Step 3: Process results.";
echo $message;

Escape Sequences (Double Quotes Only)

Strings Are Byte Sequences

In PHP, each character in a string is one byte. This works fine for ASCII text but can cause issues with multibyte characters (like emoji or accented letters). For multibyte-safe operations, use the mb_* functions:

<?php
$text = "Café";
echo strlen($text);      // 5 (the é is 2 bytes in UTF-8!)
echo mb_strlen($text);   // 4 (correct character count)

$emoji = "Hello 🌍";
echo strlen($emoji);     // 10 (🌍 is 4 bytes)
echo mb_strlen($emoji);  // 7  (correct character count)

Measuring & Searching

strlen() — String Length

<?php
echo strlen("Hello");        // 5
echo strlen("");              // 0
echo strlen("Hello World");  // 11 (space counts!)
echo strlen("   ");          // 3 (spaces are characters)

// Common use: validation
$password = "abc123";
if (strlen($password) < 8) {
    echo "Password must be at least 8 characters!\n";
}

strpos() — Find Position of Substring

<?php
$text = "The quick brown fox jumps over the lazy dog";

// Find first occurrence
echo strpos($text, "fox");     // 16
echo strpos($text, "the");     // 31 (case-sensitive! "the" ≠ "The")
echo strpos($text, "cat");     // false (not found)

// Start searching from a position
echo strpos($text, "the", 10); // 31 (start search from index 10)

// Case-insensitive search
echo stripos($text, "the");    // 0 (finds "The" at position 0)

$text = "Hello World";

// BAD — 0 is falsy, so this incorrectly says "not found"!
if (strpos($text, "Hello") == false) {
    echo "Not found!"; // WRONG — this prints!
}

// GOOD — strict comparison
if (strpos($text, "Hello") === false) {
    echo "Not found!";
} else {
    echo "Found!"; // Correct!
}

// BEST (PHP 8+) — use str_contains instead
if (str_contains($text, "Hello")) {
    echo "Found!"; // Clean and clear
}

PHP 8 String Search Functions

PHP 8 introduced three cleaner alternatives to strpos() for common checks:

<?php
$url = "https://www.example.com/products/widget";

// str_contains — does the string contain a substring?
echo str_contains($url, "example");    // true
echo str_contains($url, "Example");    // false (case-sensitive)

// str_starts_with — does it start with...?
echo str_starts_with($url, "https");   // true
echo str_starts_with($url, "http://"); // false

// str_ends_with — does it end with...?
echo str_ends_with($url, "widget");    // true
echo str_ends_with($url, ".html");     // false

// Practical examples
$filename = "report_2026.pdf";
if (str_ends_with($filename, ".pdf")) {
    echo "It's a PDF!\n";
}

$email = "alice@example.com";
if (str_contains($email, "@")) {
    echo "Looks like an email\n";
}

strrpos() — Find Last Occurrence

<?php
$path = "/home/user/documents/report.pdf";

// Find last occurrence of "/"
$lastSlash = strrpos($path, "/");
echo $lastSlash; // 20

// Useful for extracting filenames
$filename = substr($path, $lastSlash + 1);
echo $filename; // "report.pdf"

// Find last occurrence of "." for file extension
$lastDot = strrpos($path, ".");
$extension = substr($path, $lastDot + 1);
echo $extension; // "pdf"

substr_count() — Count Occurrences

<?php
$text = "she sells seashells by the seashore";

echo substr_count($text, "sea");  // 2 ("seashells" and "seashore")
echo substr_count($text, "she");  // 3 ("she", "seashells", "seashore")
echo substr_count($text, "the");  // 1

Extracting Substrings

substr() — Extract a Portion

<?php
$text = "Hello, World!";

// From position 7 to end
echo substr($text, 7);        // "World!"

// From position 7, take 5 characters
echo substr($text, 7, 5);     // "World"

// From position 0, take 5 characters
echo substr($text, 0, 5);     // "Hello"

// Negative offset — count from end
echo substr($text, -6);       // "orld!"
echo substr($text, -6, 4);    // "orld"

// Negative length — stop N characters before end
echo substr($text, 0, -1);    // "Hello, World"  (removes last char)
echo substr($text, 7, -1);    // "World"         (from 7, stop before last)

Practical Extraction Patterns

<?php
// Extract file extension
$file = "vacation_photo.jpg";
$ext = substr($file, strrpos($file, ".") + 1);
echo $ext; // "jpg"

// Or use pathinfo (the better way for file paths)
$info = pathinfo($file);
echo $info["extension"]; // "jpg"
echo $info["filename"];  // "vacation_photo"

// Truncate long text with ellipsis
function truncate(string $text, int $maxLength = 50): string {
    if (strlen($text) <= $maxLength) {
        return $text;
    }
    return substr($text, 0, $maxLength - 3) . "...";
}

echo truncate("This is a very long sentence that goes on and on", 30);
// "This is a very long senten..."

// Extract domain from email
$email = "alice@example.com";
$domain = substr($email, strpos($email, "@") + 1);
echo $domain; // "example.com"

substr_replace() — Replace a Portion

<?php
$phone = "2025551234";

// Insert dashes to format phone number
$formatted = substr_replace($phone, "-", 3, 0);  // Insert after 3
$formatted = substr_replace($formatted, "-", 7, 0); // Insert after 7
echo $formatted; // "202-555-1234"

// Mask a credit card number
$cc = "4532015112830366";
$masked = substr_replace($cc, str_repeat("*", 12), 0, 12);
echo $masked; // "************0366"

// Or show first and last 4
$masked = substr($cc, 0, 4) . str_repeat("*", 8) . substr($cc, -4);
echo $masked; // "4532********0366"

Replacing & Transforming

str_replace() — Find and Replace

<?php
$text = "I love cats. Cats are amazing. My cat is fluffy.";

// Simple replacement (case-sensitive)
echo str_replace("cat", "dog", $text);
// "I love dogs. Cats are amazing. My dog is fluffy."
// Notice: "Cats" wasn't replaced (capital C)

// Case-insensitive replacement
echo str_ireplace("cat", "dog", $text);
// "I love dogs. dogs are amazing. My dog is fluffy."

// Replace multiple values at once
$clean = str_replace(
    ["<br>", "<br/>", "<br />"],
    "\n",
    "Line 1<br>Line 2<br/>Line 3<br />Line 4"
);
// "Line 1\nLine 2\nLine 3\nLine 4"

// Map replacements (parallel arrays)
$template = "Dear {NAME}, your order #{ORDER} ships on {DATE}.";
$message = str_replace(
    ["{NAME}", "{ORDER}", "{DATE}"],
    ["Alice", "1042", "April 20, 2026"],
    $template
);
echo $message;
// "Dear Alice, your order #1042 ships on April 20, 2026."

// Count replacements made
$result = str_replace("the", "THE", $text, $count);
echo "Made $count replacements"; // Made 0 replacements (no "the" in original)

Case Conversion

<?php
$text = "Hello World";

echo strtolower($text);   // "hello world"
echo strtoupper($text);   // "HELLO WORLD"
echo ucfirst("hello");    // "Hello"     (capitalize first letter)
echo lcfirst("Hello");    // "hello"     (lowercase first letter)
echo ucwords("hello world from php");  // "Hello World From Php"

// ucwords with custom delimiters
echo ucwords("hello-world_from-php", "-_");
// "Hello-World_From-Php"

// Convert to title case (handle common words)
function title_case(string $text): string {
    $small = ["a", "an", "the", "and", "but", "or", "for", "in", "on", "at", "to", "of"];
    $words = explode(" ", strtolower($text));
    foreach ($words as $i => &$word) {
        if ($i === 0 || !in_array($word, $small)) {
            $word = ucfirst($word);
        }
    }
    return implode(" ", $words);
}

echo title_case("the lord of the rings"); // "The Lord of the Rings"

Trimming Whitespace

<?php
$input = "   Hello, World!   ";

echo trim($input);   // "Hello, World!"  (both sides)
echo ltrim($input);  // "Hello, World!   " (left only)
echo rtrim($input);  // "   Hello, World!" (right only)

// Trim specific characters
echo trim("***Hello***", "*");     // "Hello"
echo trim("...Hello...", ".");     // "Hello"
echo ltrim("/path/to/file", "/");  // "path/to/file"
echo rtrim("Hello!!!", "!");       // "Hello"

// Common use: clean form input
$email = trim($_POST["email"] ?? "");
$name  = trim($_POST["name"] ?? "");

Padding Strings

<?php
// str_pad — pad to a minimum length
echo str_pad("42", 5, "0", STR_PAD_LEFT);   // "00042"
echo str_pad("Hi", 10, ".");                  // "Hi........"
echo str_pad("Hi", 10, "-", STR_PAD_BOTH);   // "----Hi----"

// Practical: format an invoice line
function format_line(string $item, float $price): string {
    $left = str_pad($item, 30, ".");
    $right = str_pad(number_format($price, 2), 10, " ", STR_PAD_LEFT);
    return $left . $right;
}

echo format_line("Widget (x3)", 38.97) . "\n";
echo format_line("Gadget (x1)", 49.99) . "\n";
// Widget (x3)....................     38.97
// Gadget (x1)....................     49.99

Repeating Strings

<?php
echo str_repeat("=", 40) . "\n";  // ========================================
echo str_repeat("- ", 20) . "\n";  // - - - - - - - - - - - - - - - - - - - - 
echo str_repeat("Ha", 3);          // HaHaHa

// Practical: create a visual bar chart
function bar(string $label, int $value, int $max = 50): string {
    $width = (int)($value / $max * 30);
    return sprintf("%-10s |%s| %d", $label, str_repeat("█", $width), $value);
}

echo bar("PHP", 45) . "\n";
echo bar("Python", 38) . "\n";
echo bar("JS", 50) . "\n";

Splitting & Joining

explode() — String → Array

<?php
// Basic split
$csv = "Alice,Bob,Carol,Dave";
$names = explode(",", $csv);
// ["Alice", "Bob", "Carol", "Dave"]

// Split by any delimiter
$path = "/home/user/documents/file.txt";
$parts = explode("/", $path);
// ["", "home", "user", "documents", "file.txt"]

// Limit the number of splits
$log = "2026-04-17 14:30:00 ERROR Something went wrong badly";
$parts = explode(" ", $log, 3); // Split into at most 3 pieces
// ["2026-04-17", "14:30:00", "ERROR Something went wrong badly"]

// Split and destructure
[$date, $time, $message] = explode(" ", $log, 3);
echo "Date: $date, Time: $time, Message: $message\n";

// Parse key=value pairs
$queryString = "name=Alice&age=30&city=Portland";
$pairs = explode("&", $queryString);
$params = [];
foreach ($pairs as $pair) {
    [$key, $value] = explode("=", $pair);
    $params[$key] = $value;
}
// ["name" => "Alice", "age" => "30", "city" => "Portland"]

implode() — Array → String

<?php
$words = ["Hello", "World", "from", "PHP"];
echo implode(" ", $words);   // "Hello World from PHP"
echo implode(", ", $words);  // "Hello, World, from, PHP"
echo implode("", $words);    // "HelloWorldfromPHP"

// Build a CSV line
$row = ["Widget", "12.99", "50", "Tools"];
echo implode(",", $row); // "Widget,12.99,50,Tools"

// Build an HTML list
$items = ["Home", "About", "Contact"];
$html = "<li>" . implode("</li><li>", $items) . "</li>";
// "<li>Home</li><li>About</li><li>Contact</li>"

// Build a SQL IN clause
$ids = [1, 5, 12, 23];
$in = implode(",", $ids);
$sql = "SELECT * FROM users WHERE id IN ($in)";
// ⚠️ For real queries, use prepared statements! This is just to show implode.

str_split() — Split into Characters or Chunks

<?php
// Split into individual characters
$chars = str_split("Hello");
// ["H", "e", "l", "l", "o"]

// Split into chunks of N characters
$chunks = str_split("ABCDEFGHIJ", 3);
// ["ABC", "DEF", "GHI", "J"]

// Practical: format a serial number
$serial = "A1B2C3D4E5";
echo implode("-", str_split($serial, 4));
// "A1B2-C3D4-E5"

wordwrap() — Wrap Long Text

<?php
$text = "PHP is a popular general-purpose scripting language that powers everything from blogs to enterprise applications.";

// Wrap at 40 characters
echo wordwrap($text, 40, "\n");
/* Output:
PHP is a popular general-purpose
scripting language that powers
everything from blogs to enterprise
applications.
*/

// Force-cut long words
$long = "Supercalifragilisticexpialidocious is a very long word.";
echo wordwrap($long, 15, "\n", true);

Formatting Output

sprintf() — Formatted Strings

sprintf() lets you build strings with precise control over how values are formatted. It uses format specifiers as placeholders:

<?php
// Basic formatting
echo sprintf("Hello, %s! You are %d years old.\n", "Alice", 30);
// "Hello, Alice! You are 30 years old."

// Price formatting
echo sprintf("Price: $%8.2f\n", 1234.5);
// "Price: $ 1234.50"

// Table-style output
$products = [
    ["Widget", 12.99, 150],
    ["Gadget", 49.99, 30],
    ["Doohickey", 8.50, 200],
];

echo sprintf("%-15s %10s %8s\n", "Product", "Price", "Stock");
echo str_repeat("-", 35) . "\n";
foreach ($products as [$name, $price, $stock]) {
    echo sprintf("%-15s %10.2f %8d\n", $name, $price, $stock);
}
/* Output:
Product              Price    Stock
-----------------------------------
Widget               12.99      150
Gadget               49.99       30
Doohickey             8.50      200
*/

// printf — same as sprintf but prints directly
printf("Order #%05d: $%.2f\n", 42, 99.50);
// "Order #00042: $99.50"

number_format() — Format Numbers for Display

<?php
echo number_format(1234567.891);        // "1,234,568"       (rounded, no decimals)
echo number_format(1234567.891, 2);     // "1,234,567.89"    (2 decimal places)
echo number_format(1234567.891, 2, ".", ","); // "1,234,567.89" (explicit separators)
echo number_format(1234567.891, 2, ",", "."); // "1.234.567,89" (European format)
echo number_format(0.5, 2);            // "0.50"

// Practical use
$price = 49999.99;
echo "Total: $" . number_format($price, 2); // "Total: $49,999.99"

Heredoc & Nowdoc

When you need multiline strings — especially for HTML templates, SQL queries, or email bodies — heredoc and nowdoc syntax keeps your code clean.

Heredoc (Like Double Quotes)

Variables are interpolated, escape sequences work:

<?php
$name = "Alice";
$role = "Admin";

$html = <<<HTML
<div class="user-card">
    <h2>{$name}</h2>
    <p>Role: {$role}</p>
    <p>Welcome to the dashboard!</p>
</div>
HTML;

echo $html;

Nowdoc (Like Single Quotes)

No interpolation — everything is literal text. Note the single quotes around the identifier:

<?php
$code = <<<'CODE'
<?php
$name = "World";
echo "Hello, $name!";
// This $name won't be interpolated
?>
CODE;

echo $code;
// Outputs the PHP code literally, with $name as text

Practical Use Cases

<?php
// Email template with heredoc
$to = "alice@example.com";
$orderNum = 1042;
$total = 89.97;

$body = <<<EMAIL
Hi {$to},

Your order #{$orderNum} has been confirmed!

Order Total: \${$total}
Estimated Delivery: 3-5 business days

Thank you for your purchase!

Best regards,
The Store Team
EMAIL;

// SQL query with heredoc (for readability — still use prepared statements!)
$table = "products";
$sql = <<<SQL
SELECT
    p.name,
    p.price,
    c.name AS category
FROM {$table} p
JOIN categories c ON p.category_id = c.id
WHERE p.price > 10
ORDER BY p.price DESC
LIMIT 20
SQL;

Regular Expressions Basics

Regular expressions (regex) are patterns that describe text. They're powerful for validation, searching, and complex replacements. PHP uses PCRE (Perl Compatible Regular Expressions).

Essential Pattern Characters

preg_match() — Test for a Match

<?php
// Returns 1 if pattern matches, 0 if not
$email = "alice@example.com";
if (preg_match('/^[\w.+-]+@[\w-]+\.[\w.]+$/', $email)) {
    echo "Valid email format!\n";
}

// Capture groups
$date = "2026-04-17";
if (preg_match('/^(\d{4})-(\d{2})-(\d{2})$/', $date, $matches)) {
    echo "Full match: {$matches[0]}\n"; // "2026-04-17"
    echo "Year: {$matches[1]}\n";       // "2026"
    echo "Month: {$matches[2]}\n";      // "04"
    echo "Day: {$matches[3]}\n";        // "17"
}

// Named capture groups
$url = "https://example.com:8080/path";
$pattern = '/^(?P<scheme>https?):\/\/(?P<host>[\w.]+)(:(?P<port>\d+))?/';
if (preg_match($pattern, $url, $matches)) {
    echo "Scheme: {$matches['scheme']}\n"; // "https"
    echo "Host: {$matches['host']}\n";     // "example.com"
    echo "Port: {$matches['port']}\n";     // "8080"
}

preg_match_all() — Find All Matches

<?php
$text = "Call 555-1234 or 555-5678. Fax: 555-9999.";

// Find all phone numbers
$count = preg_match_all('/\d{3}-\d{4}/', $text, $matches);
echo "Found $count numbers:\n";
print_r($matches[0]); // ["555-1234", "555-5678", "555-9999"]

// Extract all hashtags
$post = "Loving the #sunset today! #photography #nature #beautiful";
preg_match_all('/#(\w+)/', $post, $matches);
print_r($matches[1]); // ["sunset", "photography", "nature", "beautiful"]

preg_replace() — Search and Replace with Regex

<?php
// Remove extra whitespace
$messy = "Hello    World   from     PHP";
$clean = preg_replace('/\s+/', ' ', $messy);
echo $clean; // "Hello World from PHP"

// Remove non-alphanumeric characters
$input = "Hello, World! @#$ 123";
$clean = preg_replace('/[^a-zA-Z0-9\s]/', '', $input);
echo $clean; // "Hello World  123"

// Format a phone number
$phone = "2025551234";
$formatted = preg_replace('/(\d{3})(\d{3})(\d{4})/', '($1) $2-$3', $phone);
echo $formatted; // "(202) 555-1234"

// Censor bad words
$text = "This is damn annoying and hell of a mess";
$censored = preg_replace('/\b(damn|hell)\b/i', '****', $text);
echo $censored; // "This is **** annoying and **** of a mess"

// Convert snake_case to camelCase
$snake = "get_user_by_id";
$camel = preg_replace_callback('/_([a-z])/', fn($m) => strtoupper($m[1]), $snake);
echo $camel; // "getUserById"

preg_split() — Split with Regex

<?php
// Split on any whitespace (spaces, tabs, multiple spaces)
$text = "Hello   World\tfrom  PHP";
$words = preg_split('/\s+/', $text);
// ["Hello", "World", "from", "PHP"]

// Split on multiple delimiters
$data = "one,two;three|four.five";
$parts = preg_split('/[,;|.]/', $data);
// ["one", "two", "three", "four", "five"]

Common Regex Patterns

💡 Tip: For email validation, prefer filter_var($email, FILTER_VALIDATE_EMAIL) over regex — it handles the full RFC spec. We'll cover this in Lesson 12 (Input Validation).

Hands-On Exercises

<?php
function analyze_text(string $text): array {
    // Clean and split into words
    $text = trim($text);
    $words = preg_split('/\s+/', $text);
    $wordCount = count($words);

    // Sentence count
    $sentenceCount = preg_match_all('/[.!?]+/', $text);

    // Word lengths
    $cleanWords = array_map(fn($w) => preg_replace('/[^a-zA-Z]/', '', $w), $words);
    $cleanWords = array_filter($cleanWords, fn($w) => strlen($w) > 0);
    $lengths = array_map('strlen', $cleanWords);
    $avgLength = count($lengths) > 0 ? array_sum($lengths) / count($lengths) : 0;

    // Longest word
    $longest = array_reduce($cleanWords,
        fn($carry, $w) => strlen($w) > strlen($carry) ? $w : $carry, "");

    // Unique words (case-insensitive)
    $lowerWords = array_map('strtolower', $cleanWords);
    $uniqueCount = count(array_unique($lowerWords));

    return [
        "char_count"      => strlen($text),
        "word_count"       => $wordCount,
        "sentence_count"   => $sentenceCount,
        "avg_word_length"  => round($avgLength, 1),
        "longest_word"     => $longest,
        "unique_words"     => $uniqueCount,
    ];
}

// Test it
$sample = "PHP is a popular scripting language. It powers millions of websites! 
Do you know PHP? PHP is great for web development. Learning PHP is fun and rewarding.";

$stats = analyze_text($sample);
echo "📊 Text Analysis\n";
echo str_repeat("=", 30) . "\n";
foreach ($stats as $key => $value) {
    echo sprintf("  %-18s %s\n", str_replace("_", " ", ucfirst($key)) . ":", $value);
}

<?php
function render_template(string $template, array $data): string {
    return preg_replace_callback(
        '/\{\{(\w+)(?:\|(\w+))?\}\}/',
        function($matches) use ($data) {
            $key = $matches[1];
            $filter = $matches[2] ?? null;

            // Get value or show missing
            if (!array_key_exists($key, $data)) {
                return "[MISSING: $key]";
            }

            $value = (string) $data[$key];

            // Apply filter
            return match ($filter) {
                'upper' => strtoupper($value),
                'lower' => strtolower($value),
                'ucfirst' => ucfirst($value),
                'trim' => trim($value),
                null => $value,
                default => $value,
            };
        },
        $template
    );
}

// Test it
$template = <<<TPL
Dear {{name|ucfirst}},

Thank you for your order #{{order_id}}!

Items: {{items}}
Total: \${{total}}
Status: {{status|upper}}

Shipping to: {{address}}
Expected delivery: {{delivery_date}}

Best regards,
{{company|upper}} Team
TPL;

$data = [
    "name"          => "alice",
    "order_id"      => "1042",
    "items"         => "Widget x3, Gadget x1",
    "total"         => "88.96",
    "status"        => "confirmed",
    "address"       => "123 Main St, Portland, OR",
    "company"       => "Acme Widgets",
];

echo render_template($template, $data);

slugify("Hello World!")          → "hello-world"
slugify("PHP 8.3: What's New?")  → "php-8-3-whats-new"
slugify("  Extra   Spaces  ")    → "extra-spaces"
slugify("CamelCase & snake_case") → "camelcase-snake-case"

<?php
function slugify(string $text): string {
    $text = strtolower(trim($text));              // Lowercase & trim
    $text = preg_replace('/[^a-z0-9\s-]/', '', $text); // Remove special chars
    $text = preg_replace('/[\s_]+/', '-', $text);       // Spaces/underscores → hyphens
    $text = preg_replace('/-+/', '-', $text);           // Collapse multiple hyphens
    $text = trim($text, '-');                            // Trim leading/trailing hyphens
    return $text;
}

// Test
$titles = [
    "Hello World!",
    "PHP 8.3: What's New?",
    "  Extra   Spaces  ",
    "CamelCase & snake_case",
    "10 Tips for Better Code!!!",
    "---Leading & Trailing---",
];

foreach ($titles as $title) {
    echo sprintf("  %-35s → %s\n", "\"$title\"", slugify($title));
}

Summary

📚 Additional Resources

• PHP Manual: String Functions
• PHP Manual: Multibyte String Functions
• PHP Manual: PCRE Pattern Syntax
• regex101.com — Interactive Regex Tester
• PHP Manual: sprintf()

🚀 What's Next?

With arrays and strings mastered, you're ready to start building real web applications. In Lesson 11: Working with Forms, you'll learn how to receive user input through HTML forms, handle $_GET and $_POST data, process multi-value inputs, and create sticky forms.