PHP Fundamentals

Strings & String Functions

13 min Lesson 5 of 45

Strings & String Functions

Strings are one of the most commonly used data types in PHP. In this lesson, we'll explore how to create, manipulate, and transform strings using PHP's powerful built-in functions.

Creating Strings

PHP provides multiple ways to create strings:

<?php
    // Single quotes - literal strings
    $name = 'John Doe';
    $path = 'C:\xampp\htdocs';  // No escape needed

    // Double quotes - interprets variables and escape sequences
    $greeting = "Hello, $name";
    $message = "Welcome\nTo PHP";  // \n is newline

    // Curly braces for complex variables
    $user = ['name' => 'John'];
    echo "User: {$user['name']}";

    // Heredoc - multi-line strings with parsing
    $html = <<<HTML
    <div class="container">
        <h1>Welcome $name</h1>
        <p>This is a multi-line string</p>
    </div>
    HTML;

    // Nowdoc - multi-line strings without parsing (like single quotes)
    $code = <<<'CODE'
    <?php
        echo "This won't be parsed: $variable";
    ?>
    CODE;
?>

Best Practice: Use single quotes for static strings (faster). Use double quotes only when you need variable interpolation or escape sequences.

Escape Sequences

Special characters in double-quoted strings:

Common Escape Sequences:

\n    - Newline
\r    - Carriage return
\t    - Tab
\\    - Backslash
\"    - Double quote
\$    - Dollar sign (prevent variable parsing)
\'    - Single quote (needed in single-quoted strings)

Examples:
<?php
    echo "Line 1\nLine 2";          // Two lines
    echo "Name:\tJohn";             // Tab between Name: and John
    echo "Price: \$19.99";          // Literal dollar sign
    echo 'It\'s a beautiful day'; // Escaped single quote
?>

String Concatenation

Combining strings using the dot (.) operator:

<?php
    $firstName = "John";
    $lastName = "Doe";

    // Basic concatenation
    $fullName = $firstName . " " . $lastName;

    // Concatenation with assignment
    $message = "Hello";
    $message .= ", ";
    $message .= $firstName;

    // Multiple concatenations
    $html = "<div>" .
            "<h1>" . $fullName . "</h1>" .
            "<p>Welcome!</p>" .
            "</div>";

    // Mixed with other types
    $age = 25;
    $info = "Name: " . $fullName . ", Age: " . $age;
?>

String Length

Get the number of characters in a string:

<?php
    $text = "Hello World";
    $length = strlen($text);
    echo $length;  // Output: 11

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

    // Multi-byte strings (UTF-8)
    $arabic = "مرحبا";
    echo strlen($arabic);      // Byte count (may be wrong)
    echo mb_strlen($arabic);   // Character count (correct)
?>

Important: Use mb_strlen() for multi-byte characters (Arabic, Chinese, emojis) instead of strlen() to get accurate character counts.

Case Conversion

Change the case of strings:

<?php
    $text = "Hello World";

    // Convert to lowercase
    echo strtolower($text);    // Output: hello world

    // Convert to uppercase
    echo strtoupper($text);    // Output: HELLO WORLD

    // Capitalize first letter
    echo ucfirst($text);       // Output: Hello world

    // Capitalize first letter of each word
    echo ucwords($text);       // Output: Hello World

    // Lowercase first letter
    echo lcfirst("Hello");     // Output: hello

    // Practical use: email normalization
    $email = "JohnDoe@EXAMPLE.COM";
    $normalized = strtolower($email);  // johndoe@example.com
?>

Trimming Whitespace

Remove whitespace from the beginning and/or end of strings:

<?php
    $text = "  Hello World  ";

    // Remove from both sides
    echo trim($text);      // "Hello World"

    // Remove from left side only
    echo ltrim($text);     // "Hello World  "

    // Remove from right side only
    echo rtrim($text);     // "  Hello World"

    // Remove specific characters
    $url = "///example.com///";
    echo trim($url, "/");  // "example.com"

    // Practical use: clean user input
    $username = trim($_POST['username'] ?? '');
?>

Best Practice: Always trim user input from forms to remove accidental spaces that users might enter.

Finding Substrings

Search for text within strings:

<?php
    $text = "Hello World, Welcome to PHP World";

    // Find first occurrence (returns position or false)
    $pos = strpos($text, "World");
    echo $pos;  // Output: 6

    // Find last occurrence
    $lastPos = strrpos($text, "World");
    echo $lastPos;  // Output: 32

    // Case-insensitive search
    $pos = stripos($text, "world");
    echo $pos;  // Output: 6

    // Check if string contains substring
    if (strpos($text, "PHP") !== false) {
        echo "Found PHP";
    }

    // PHP 8+ (recommended)
    if (str_contains($text, "PHP")) {
        echo "Found PHP";
    }

    // Starts with / Ends with (PHP 8+)
    if (str_starts_with($text, "Hello")) {
        echo "Starts with Hello";
    }

    if (str_ends_with($text, "World")) {
        echo "Ends with World";
    }
?>

Important: Always use !== false when checking strpos() results, because position 0 evaluates to false in a boolean context.

Extracting Substrings

Get portions of a string:

<?php
    $text = "Hello World";

    // Extract from position to end
    echo substr($text, 6);        // "World"

    // Extract with length
    echo substr($text, 0, 5);     // "Hello"

    // Negative offset (from end)
    echo substr($text, -5);       // "World"

    // Negative length (exclude from end)
    echo substr($text, 0, -6);    // "Hello"

    // Extract middle portion
    echo substr($text, 3, 5);     // "lo Wo"

    // Practical use: truncate text
    $description = "This is a very long description...";
    $short = substr($description, 0, 50) . "...";
?>

Replacing Text

Find and replace text within strings:

<?php
    $text = "Hello World";

    // Replace all occurrences
    echo str_replace("World", "PHP", $text);
    // Output: Hello PHP

    // Case-insensitive replace
    echo str_ireplace("world", "PHP", $text);
    // Output: Hello PHP

    // Multiple replacements
    $text = "I like apples and oranges";
    $search = ["apples", "oranges"];
    $replace = ["bananas", "grapes"];
    echo str_replace($search, $replace, $text);
    // Output: I like bananas and grapes

    // Count replacements
    $count = 0;
    $result = str_replace("a", "X", "banana", $count);
    echo $result;  // Output: bXnXnX
    echo $count;   // Output: 3

    // Replace first occurrence only
    $text = "foo bar foo bar";
    echo preg_replace('/foo/', 'baz', $text, 1);
    // Output: baz bar foo bar
?>

Splitting and Joining Strings

Convert between strings and arrays:

<?php
    // Split string into array
    $text = "apple,banana,orange";
    $fruits = explode(",", $text);
    print_r($fruits);
    // Array([0]=>apple [1]=>banana [2]=>orange)

    // Join array into string
    $joined = implode(", ", $fruits);
    echo $joined;  // Output: apple, banana, orange

    // Alternative: join() (alias of implode)
    echo join(" - ", $fruits);

    // Split with limit
    $text = "a:b:c:d:e";
    $parts = explode(":", $text, 3);
    // Array([0]=>a [1]=>b [2]=>c:d:e)

    // Practical use: parse CSV
    $csv = "John,Doe,john@example.com";
    list($firstName, $lastName, $email) = explode(",", $csv);
?>

Remember: explode() creates array from string. implode() creates string from array. Think: "explode" breaks apart, "implode" brings together.

String Comparison

Compare strings in various ways:

<?php
    $str1 = "apple";
    $str2 = "Apple";
    $str3 = "banana";

    // Case-sensitive comparison
    echo strcmp($str1, $str2);     // > 0 (different case)
    echo strcmp($str1, $str1);     // 0 (equal)
    echo strcmp($str1, $str3);     // < 0 (apple before banana)

    // Case-insensitive comparison
    echo strcasecmp($str1, $str2); // 0 (same ignoring case)

    // Natural order comparison (human-friendly)
    $files = ["file10.txt", "file2.txt", "file1.txt"];
    usort($files, "strnatcmp");
    // Result: file1.txt, file2.txt, file10.txt

    // Compare first n characters
    echo strncmp("hello", "help", 3);  // 0 ("hel" equals "hel")
?>

String Formatting

Format strings with placeholders:

<?php
    // sprintf - return formatted string
    $name = "John";
    $age = 25;
    $text = sprintf("Name: %s, Age: %d", $name, $age);
    echo $text;  // Output: Name: John, Age: 25

    // printf - echo formatted string
    printf("Name: %s, Age: %d", $name, $age);

    // Common format specifiers:
    // %s - string
    // %d - integer
    // %f - float
    // %b - binary
    // %x - hexadecimal

    // Format decimal places
    $price = 19.9567;
    echo sprintf("Price: $%.2f", $price);
    // Output: Price: $19.96

    // Padding
    echo sprintf("%05d", 42);      // Output: 00042
    echo sprintf("%-10s", "test"); // Output: "test      "
?>

HTML and URL Functions

Handle HTML and URLs safely:

<?php
    // HTML special characters
    $text = "<script>alert('XSS')</script>";
    echo htmlspecialchars($text);
    // Output: &lt;script&gt;alert('XSS')&lt;/script&gt;

    // Strip HTML tags
    $html = "<p>Hello <b>World</b></p>";
    echo strip_tags($html);        // Output: Hello World
    echo strip_tags($html, '<b>'); // Output: Hello <b>World</b>

    // URL encoding
    $query = "search term with spaces";
    echo urlencode($query);
    // Output: search+term+with+spaces

    // URL decoding
    $encoded = "search%20term";
    echo urldecode($encoded);  // Output: search term

    // Raw URL encoding (for URLs, not query strings)
    echo rawurlencode("path/to/file name.txt");
    // Output: path%2Fto%2Ffile%20name.txt
?>

Security: Always use htmlspecialchars() when displaying user-generated content in HTML to prevent XSS attacks!

Other Useful String Functions

<?php
    // Reverse a string
    echo strrev("Hello");  // Output: olleH

    // Repeat a string
    echo str_repeat("*", 10);  // Output: **********

    // Shuffle string characters
    echo str_shuffle("Hello");  // Output: random order

    // Count substring occurrences
    $text = "Hello World, Welcome to PHP World";
    echo substr_count($text, "World");  // Output: 2

    // Word count
    echo str_word_count("Hello World");  // Output: 2

    // Number formatting
    echo number_format(1234567.89);        // 1,234,568
    echo number_format(1234567.89, 2);     // 1,234,567.89
    echo number_format(1234567.89, 2, '.', ',');
    // 1,234,567.89

    // Parse string to variables
    parse_str("name=John&age=25", $data);
    print_r($data);
    // Array([name]=>John [age]=>25)
?>

Multi-byte String Functions

For international character support (UTF-8):

<?php
    $text = "مرحباً بك في PHP";

    // Multi-byte string length
    echo mb_strlen($text);

    // Multi-byte substring
    echo mb_substr($text, 0, 5);

    // Multi-byte case conversion
    echo mb_strtoupper($text);
    echo mb_strtolower($text);

    // Multi-byte strpos
    echo mb_strpos($text, "PHP");

    // Set default encoding
    mb_internal_encoding("UTF-8");
?>

Practice Exercise:

Task: Create a string manipulation script that:

Defines a sentence: " PHP is a POWERFUL server-side language "
Trims whitespace
Converts to title case (first letter of each word uppercase)
Counts the number of words
Replaces "PHP" with "PHP 8"
Extracts the first 20 characters
Displays all results

Solution:

<?php
    // Original string
    $sentence = "  PHP is a POWERFUL server-side language  ";

    // Trim whitespace
    $trimmed = trim($sentence);
    echo "<p>Trimmed: '$trimmed'</p>";

    // Convert to title case
    $titleCase = ucwords(strtolower($trimmed));
    echo "<p>Title Case: $titleCase</p>";

    // Count words
    $wordCount = str_word_count($titleCase);
    echo "<p>Word Count: $wordCount</p>";

    // Replace PHP with PHP 8
    $replaced = str_replace("Php", "PHP 8", $titleCase);
    echo "<p>Replaced: $replaced</p>";

    // Extract first 20 characters
    $excerpt = substr($replaced, 0, 20);
    echo "<p>First 20 chars: $excerpt...</p>";

    // Display length
    echo "<p>Length: " . strlen($replaced) . " characters</p>";
?>

Summary

In this lesson, you learned:

Create strings using single quotes, double quotes, heredoc, and nowdoc
Use strlen() and mb_strlen() to get string length
Convert case with strtolower(), strtoupper(), ucfirst(), ucwords()
Trim whitespace with trim(), ltrim(), rtrim()
Search strings with strpos(), stripos(), str_contains()
Extract substrings with substr()
Replace text with str_replace(), str_ireplace()
Split and join with explode() and implode()
Format strings with sprintf() and printf()
Handle HTML safely with htmlspecialchars()
Use multi-byte functions for international characters

Great Progress! You've completed Module 1: PHP Basics & Environment Setup. You now understand PHP syntax, variables, operators, and strings. Next, we'll dive into control structures like if-else, loops, and functions!