How To Add Key-Value Pairs To An Associative Array In PHP?

Method 1: Direct Assignment

To add a key-value pair to an associative array using direct assignment, use this syntax:

$array_name['key'] = 'value';

Example of adding a single key-value pair:

$fruits = array();
$fruits['apple'] = 'red';

print_r($fruits);
// Output: Array ( [apple] => red )

Method 2: Using the array() Function

You can use the array() function to create an associative array with multiple key-value pairs:

$array_name = array('key1' => 'value1', 'key2' => 'value2');

Example of adding multiple key-value pairs:

$colors = array('sky' => 'blue', 'grass' => 'green', 'sun' => 'yellow');

print_r($colors);
// Output: Array ( [sky] => blue [grass] => green [sun] => yellow )

Method 3: Using the Square Bracket Notation

The square bracket notation lets you add key-value pairs dynamically:

$array_name[] = 'value';

This method is useful when you want to add elements to an array without specifying the key:

$numbers = array();
$numbers[] = 10;
$numbers[] = 20;
$numbers[] = 30;

print_r($numbers);
// Output: Array ( [0] => 10 [1] => 20 [2] => 30 )

You can also use this method with string keys:

$user = array();
$user['name'] = 'John';
$user['age'] = 25;

print_r($user);
// Output: Array ( [name] => John [age] => 25 )

$person = array('name' => 'Alice');
$person['age'] = 30;
$person[] = 'developer';

print_r($person);
// Output: Array ( [name] => Alice [age] => 30 [0] => developer )

These methods offer ways to add key-value pairs to associative arrays in PHP, allowing you to choose the best approach for your needs.

Modifying Existing Key-Value Pairs

When working with associative arrays in PHP, you may need to modify existing key-value pairs. Here are two common scenarios:

Overwriting existing values: To overwrite an existing value, assign a new value to the same key:

$fruits = ['apple' => 'red', 'banana' => 'yellow'];
$fruits['apple'] = 'green';

print_r($fruits);
// Output: Array ( [apple] => green [banana] => yellow )

Updating values based on conditions: You can update values based on conditions using if statements or ternary operators:

$scores = ['math' => 80, 'science' => 75];

if ($scores['math'] < 85) {
    $scores['math'] += 5;
}

$scores['science'] = ($scores['science'] < 70) ? 70 : $scores['science'];

print_r($scores);
// Output: Array ( [math] => 85 [science] => 75 )

$prices = ['apple' => 0.50, 'banana' => 0.75, 'orange' => 0.60];

array_walk($prices, function(&$value, $key) {
    $value *= 1.1; // Increase all prices by 10%
});

print_r($prices);
// Output: Array ( [apple] => 0.55 [banana] => 0.825 [orange] => 0.66 )

Checking for Existing Keys

Before modifying or accessing array elements, it's useful to check if a key exists. PHP provides two main functions for this purpose:

Using the isset() function: The isset() function checks if a key is set and not null:

$fruits = ['apple' => 'red', 'banana' => 'yellow'];

if (isset($fruits['apple'])) {
    echo "The 'apple' key exists.";
} else {
    echo "The 'apple' key does not exist.";
}
// Output: The 'apple' key exists.

Using the array_key_exists() function: The array_key_exists() function checks if a key exists in an array, even if its value is null:

$fruits = ['apple' => 'red', 'banana' => null];

if (array_key_exists('banana', $fruits)) {
    echo "The 'banana' key exists.";
} else {
    echo "The 'banana' key does not exist.";
}
// Output: The 'banana' key exists.

The main difference between isset() and array_key_exists() is that isset() returns false for null values, while array_key_exists() returns true even if the value is null.

Using the array_merge() Function

The array_merge() function combines two or more associative arrays. Here's the syntax:

$result = array_merge($array1, $array2, $array3, ...);

Example of merging two associative arrays:

$fruits = ['apple' => 'red', 'banana' => 'yellow'];
$vegetables = ['carrot' => 'orange', 'spinach' => 'green'];

$combined = array_merge($fruits, $vegetables);

print_r($combined);
// Output: Array ( [apple] => red [banana] => yellow [carrot] => orange [spinach] => green )

If there are duplicate keys, the values from later arrays replace the values from earlier arrays.

Using the + Operator

The + operator is another way to combine associative arrays. The syntax is:

$result = $array1 + $array2 + $array3 + ...;

Example of combining associative arrays with the + operator:

$colors1 = ['sky' => 'blue', 'grass' => 'green'];
$colors2 = ['sun' => 'yellow', 'grass' => 'brown'];

$combined = $colors1 + $colors2;

print_r($combined);
// Output: Array ( [sky] => blue [grass] => green [sun] => yellow )

The + operator keeps the first occurrence of duplicate keys, unlike array_merge() which uses the last occurrence.