Strings & Arrays

Strings and arrays are two of the most used data types in PHP. You will work with them constantly -- building messages, processing user input, storing lists of data, and transforming collections. This chapter gives you a solid foundation in both.

Strings

You already met strings in chapter 2. Here we go deeper: concatenation, heredoc and nowdoc, and the most useful string functions.

String recap -- single vs double quotes

Recall from chapter 2:

• Single quotes ('...'): Literal text. Variables and most escape sequences are not interpreted.
• Double quotes ("..."): Variables are interpolated, and escape sequences like \n and \t work.

<?php

$name = 'Alice';

echo 'Hello, $name!';   // Output: Hello, $name!  (literal)
echo "Hello, $name!";   // Output: Hello, Alice!  (interpolated)

Tip: Use single quotes when you have plain text with no variables. Use double quotes when you need variable interpolation or escape sequences.

String concatenation

The dot operator (.) joins strings together. You can chain it to build longer strings:

<?php

$first = 'Hello';
$second = 'World';
$greeting = $first . ', ' . $second . '!';

echo $greeting;  // Output: Hello, World!

The concatenation assignment operator (.=) appends to an existing string:

<?php

$message = 'Hello';
$message .= ', ';
$message .= 'World';
$message .= '!';

echo $message;  // Output: Hello, World!

This is useful when you build a string piece by piece in a loop or conditional:

<?php

$output = '';
$items = ['apple', 'banana', 'cherry'];

foreach ($items as $item) {
    $output .= '- ' . $item . "\n";
}

echo $output;
// Output:
// - apple
// - banana
// - cherry

Heredoc and nowdoc syntax

When you have long blocks of text with many variables, heredoc and nowdoc keep your code readable.

Heredoc behaves like double-quoted strings -- variables are interpolated:

<?php

$name = 'Alice';
$role = 'Developer';

$text = <<<EOT
Hello, $name!

You have been assigned the role of $role.
Please complete the onboarding form by Friday.

Best regards,
The Team
EOT;

echo $text;

The closing identifier (EOT in this example) must be on its own line, at the start of the line, with no spaces before it, and followed by a semicolon. You can use any identifier -- EOT, END, HTML, etc. -- as long as it matches at the start and end.

Nowdoc behaves like single-quoted strings -- no variable interpolation:

<?php

$template = <<<'EOT'
This is a literal string.
No $variables are expanded here.
Use nowdoc for templates or SQL that should stay exactly as written.
EOT;

echo $template;

The only difference is that the opening identifier is wrapped in single quotes: <<<'EOT'.

Note: Heredoc and nowdoc are especially useful for embedding HTML, SQL, or multi-line text without escaping quotes everywhere.

Common string functions

PHP has dozens of string functions. Here are the ones you will use most often:

Function	Description	Example
strlen($str)	Number of bytes (not characters) in string	strlen('Hello') → 5
strtolower($str)	Convert to lowercase	strtolower('HELLO') → 'hello'
strtoupper($str)	Convert to uppercase	strtoupper('hello') → 'HELLO'
trim($str)	Remove whitespace from both ends	trim(' hi ') → 'hi'
ltrim($str)	Remove whitespace from left	ltrim(' hi') → 'hi'
rtrim($str)	Remove whitespace from right	rtrim('hi ') → 'hi'
str_replace($search, $replace, $str)	Replace all occurrences	str_replace('a', 'o', 'cat') → 'cot'
strpos($haystack, $needle)	Position of first occurrence (or false)	strpos('hello', 'l') → 2
substr($str, $start, $length)	Extract a substring	substr('hello', 1, 3) → 'ell'
explode($delimiter, $str)	Split string into array by delimiter	explode(',', 'a,b,c') → ['a','b','c']
implode($glue, $arr)	Join array elements into string	implode('-', [1,2,3]) → '1-2-3'
str_contains($haystack, $needle)	True if string contains substring (PHP 8+)	str_contains('hello', 'ell') → true
str_starts_with($haystack, $needle)	True if string starts with substring (PHP 8+)	str_starts_with('hello', 'he') → true
str_ends_with($haystack, $needle)	True if string ends with substring (PHP 8+)	str_ends_with('hello', 'lo') → true
sprintf($format, ...$args)	Build string from format and values	sprintf('Hi %s', 'Alice') → 'Hi Alice'

Examples:

<?php

$text = '  Hello, World!  ';
echo strlen(trim($text));        // 13 (trimmed length)
echo strtolower($text);          // "  hello, world!  "
echo str_replace('World', 'PHP', $text);  // "  Hello, PHP!  "

$csv = 'apple,banana,cherry';
$fruits = explode(',', $csv);    // ['apple', 'banana', 'cherry']
echo implode(' | ', $fruits);    // "apple | banana | cherry"

if (str_contains($text, 'World')) {
    echo 'Found World!';
}

$name = 'Alice';
$age = 30;
echo sprintf('Name: %s, Age: %d', $name, $age);  // "Name: Alice, Age: 30"

Common sprintf placeholders: %s (string), %d (integer), %f (float), %.2f (float with 2 decimals).

Warning: strpos() returns 0 when the needle is at the start of the string. Since 0 is falsy in PHP, always use strict comparison: if (strpos($haystack, $needle) !== false).

Multibyte strings -- a note on Unicode

strlen() counts bytes, not characters. For ASCII text that is fine. For Unicode (e.g. emoji, accented characters, Chinese), one character can use multiple bytes:

<?php

$emoji = '👋';
echo strlen($emoji);  // 4 (bytes) -- not 1 character!

For Unicode-aware string handling, use the mbstring extension:

Function	Multibyte equivalent
strlen()	mb_strlen($str, 'UTF-8')
strpos()	mb_strpos($haystack, $needle, 0, 'UTF-8')
substr()	mb_substr($str, $start, $length, 'UTF-8')
strtolower()	mb_strtolower($str, 'UTF-8')
strtoupper()	mb_strtoupper($str, 'UTF-8')

Tip: If your application handles user input in multiple languages or emoji, prefer the mb_* functions. The mbstring extension is usually enabled by default in PHP.

Arrays

An array is an ordered collection of values. You can store numbers, strings, booleans, other arrays, or a mix. Arrays are one of PHP's most powerful features.

What is an array?

Think of an array as a list or a row of boxes. Each box holds a value, and each box has a position (or a label) so you can access it. Arrays can grow and shrink as you add or remove elements.

Indexed arrays

An indexed array uses numeric keys starting from 0. You create one with square brackets or the array() function:

<?php

$numbers = [1, 2, 3, 4, 5];
$colors = array('red', 'green', 'blue');

// Both are equivalent. The short syntax [] is preferred in modern PHP.

You access elements by their index (position). Indexing is zero-based -- the first element is at index 0:

<?php

$fruits = ['apple', 'banana', 'cherry'];

echo $fruits[0];   // apple
echo $fruits[1];   // banana
echo $fruits[2];   // cherry

Adding and removing elements

Append to the end with $arr[] = value or array_push():

<?php

$items = ['a', 'b'];
$items[] = 'c';           // ['a', 'b', 'c']
array_push($items, 'd');  // ['a', 'b', 'c', 'd']

Remove from the end with array_pop():

<?php

$stack = [1, 2, 3];
$last = array_pop($stack);   // $last is 3, $stack is [1, 2]

Remove from the beginning with array_shift():

<?php

$queue = ['first', 'second', 'third'];
$first = array_shift($queue);  // $first is 'first', $queue is ['second', 'third']

Add to the beginning with array_unshift():

<?php

$list = ['b', 'c'];
array_unshift($list, 'a');  // ['a', 'b', 'c']

Remove a specific element with unset():

<?php

$arr = [10, 20, 30];
unset($arr[1]);   // Removes index 1; $arr is [10, 30]
// Note: indices are NOT renumbered. $arr[0] is 10, $arr[2] is 30.

Note: unset() removes the key entirely. The array does not automatically reindex. Use array_values() to reindex: $arr = array_values($arr);

Associative arrays

An associative array uses string (or integer) keys that you choose. Instead of positions, you use meaningful names:

<?php

$user = [
    'name' => 'Alice',
    'email' => 'alice@example.com',
    'age' => 30,
];

echo $user['name'];   // Alice
echo $user['email'];  // alice@example.com

Use associative arrays when each value has a clear label -- user data, configuration, key-value pairs. Use indexed arrays when order matters and you do not need named keys.

Multidimensional arrays

Arrays can contain other arrays. That gives you multidimensional arrays -- arrays of arrays:

<?php

$students = [
    ['name' => 'Alice', 'grade' => 85],
    ['name' => 'Bob', 'grade' => 92],
    ['name' => 'Carol', 'grade' => 78],
];

echo $students[0]['name'];   // Alice
echo $students[1]['grade'];  // 92

You can nest as deeply as you need. Access nested values by chaining indices: $data['users'][0]['email'].

Looping through arrays

foreach is the standard way to iterate over arrays. Two forms:

By value only:

<?php

$fruits = ['apple', 'banana', 'cherry'];

foreach ($fruits as $fruit) {
    echo $fruit . "\n";
}
// apple
// banana
// cherry

By key and value:

<?php

$scores = ['Alice' => 85, 'Bob' => 92, 'Carol' => 78];

foreach ($scores as $name => $score) {
    echo "$name: $score\n";
}
// Alice: 85
// Bob: 92
// Carol: 78

for loop with count() when you need the index:

<?php

$items = ['a', 'b', 'c'];

for ($i = 0; $i < count($items); $i++) {
    echo "Index $i: " . $items[$i] . "\n";
}

Tip: Prefer foreach when you do not need the numeric index. It is clearer and avoids off-by-one errors.

Essential array functions

Function	Description	Example
count($arr)	Number of elements	count([1,2,3]) → 3
in_array($needle, $arr)	True if value exists in array	in_array('b', ['a','b','c']) → true
array_key_exists($key, $arr)	True if key exists	array_key_exists('x', ['x'=>1]) → true
array_merge($arr1, $arr2)	Merge arrays (values appended)	array_merge([1,2],[3,4]) → [1,2,3,4]
array_slice($arr, $offset, $len)	Extract portion of array	array_slice([1,2,3,4], 1, 2) → [2,3]
array_splice($arr, $offset, $len, $replacement)	Remove/replace portion	Modifies array in place
array_map($callback, $arr)	Apply function to each element	array_map('strtoupper', ['a','b']) → ['A','B']
array_filter($arr, $callback)	Keep only elements where callback is truthy	array_filter([1,0,2,0,3]) → [1,2,3]
array_reduce($arr, $callback, $initial)	Reduce to single value	Sum, product, concatenation
sort($arr)	Sort by value (reindexes)	Modifies in place
asort($arr)	Sort by value (keeps keys)	Modifies in place
ksort($arr)	Sort by key	Modifies in place
rsort($arr)	Sort by value, descending	Modifies in place
array_unique($arr)	Remove duplicate values	Returns new array
array_reverse($arr)	Reverse order	Returns new array
array_keys($arr)	Get all keys	array_keys(['a'=>1,'b'=>2]) → ['a','b']
array_values($arr)	Get all values (reindex)	array_values(['x'=>1,'y'=>2]) → [1,2]

Examples:

<?php

$nums = [3, 1, 4, 1, 5];
sort($nums);           // [1, 1, 3, 4, 5]
$unique = array_unique($nums);  // [1, 3, 4, 5]

$names = ['alice', 'bob', 'carol'];
$upper = array_map('strtoupper', $names);  // ['ALICE', 'BOB', 'CAROL']

$scores = [10, 20, 30];
$sum = array_reduce($scores, fn($carry, $n) => $carry + $n, 0);  // 60

Destructuring with list() and short syntax

You can unpack array values into variables in one step:

<?php

$point = [10, 20];
list($x, $y) = $point;
echo "$x, $y";  // 10, 20

// Short syntax (PHP 7.1+):
[$x, $y] = $point;

With associative arrays, use the key names:

<?php

$user = ['name' => 'Alice', 'email' => 'alice@example.com'];
['name' => $name, 'email' => $email] = $user;
echo $name;   // Alice
echo $email;  // alice@example.com

The spread operator in arrays

PHP 7.4+ supports the spread operator (...) to merge arrays or insert elements:

<?php

$first = [1, 2, 3];
$second = [4, 5, 6];
$merged = [...$first, ...$second];  // [1, 2, 3, 4, 5, 6]

$middle = ['x', 'y'];
$all = [0, ...$middle, 9];  // [0, 'x', 'y', 9]

This is cleaner than array_merge() when you want to combine arrays or inject elements at specific positions.

Practical example -- shopping list and gradebook

Here is a complete example that uses strings and arrays together. It builds a shopping list, manipulates it, and then processes a simple gradebook:

<?php

// --- Shopping list ---
$shoppingList = ['milk', 'bread', 'eggs'];
$shoppingList[] = 'butter';
$shoppingList[] = 'cheese';

// Remove 'eggs' (find position and unset)
$index = array_search('eggs', $shoppingList);
if ($index !== false) {
    unset($shoppingList[$index]);
    $shoppingList = array_values($shoppingList);
}

// Build a formatted string
$listStr = implode(', ', $shoppingList);
echo "Shopping list: $listStr\n";

// --- Student gradebook ---
$grades = [
    ['name' => 'Alice', 'scores' => [85, 90, 88]],
    ['name' => 'Bob', 'scores' => [78, 82, 80]],
    ['name' => 'Carol', 'scores' => [92, 95, 90]],
];

foreach ($grades as $student) {
    $name = $student['name'];
    $scores = $student['scores'];
    $average = array_reduce($scores, fn($c, $n) => $c + $n, 0) / count($scores);
    $average = round($average, 1);
    echo sprintf("%s: %.1f average\n", $name, $average);
}

Output:

Shopping list: milk, bread, butter, cheese
Alice: 87.7 average
Bob: 80.0 average
Carol: 92.3 average

This example uses indexed and associative arrays, foreach, array_search, array_values, implode, array_reduce, count, round, and sprintf -- all concepts from this chapter.

Summary

• Strings: Use single quotes for literals, double quotes for interpolation. Concatenate with . and .=
• Heredoc (<<<EOT) interpolates variables; nowdoc (<<<'EOT') does not
• Essential string functions: strlen, strtolower, strtoupper, trim, str_replace, strpos, substr, explode, implode, str_contains, str_starts_with, str_ends_with, sprintf
• For Unicode, use mb_strlen, mb_substr, and other mb_* functions
• Arrays: Indexed arrays use numeric keys (0-based); associative arrays use named keys
• Add/remove: $arr[] = x, array_push, array_pop, array_shift, array_unshift, unset
• Loop with foreach ($arr as $value) or foreach ($arr as $key => $value)
• Essential array functions: count, in_array, array_key_exists, array_merge, array_slice, array_map, array_filter, array_reduce, sort, asort, ksort, array_unique, array_keys, array_values
• Destructure with [$a, $b] = $arr or ['key' => $var] = $arr
• Spread operator: [...$arr1, ...$arr2] merges arrays

Next up: Working with Forms & HTTP -- HTML forms, GET and POST requests, superglobals, input validation, and building a contact form.