Given a 2D array where each element is an array itself, your task is to switch the first element of each sub-array with the first element of the last sub-array using PHP.

Example:

Input:  num = [
    ['a', 'b', 'c'],
    ['d', 'e', 'f'],
    ['g', 'h', 'i']
];

Output:  [
    ['g', 'b', 'c'],
    ['d', 'e', 'f'],
    ['a', 'h', 'i']
]

Using a Temporary Variable

In this approach, we store the first element of the first sub-array in a temporary variable and then replace the first element of the first sub-array with the first element of the last sub-array and at last assign the value stored in the temporary variable to the first element of the last sub-array.

Example: Below is the implementation of the above approach to switch the first element of each sub-array in a multidimensional array.

<?php

// Define the array
$array = [
    ['a', 'b', 'c'],
    ['d', 'e', 'f'],
    ['g', 'h', 'i']
];

// Check if there are at least two sub-arrays
if (count($array) >= 2) {
    // Store the first element
    // of the first sub-array
    $temp = $array[0][0];

    // Switch the first element of the first
    // sub-array with the first 
    // element of the last sub-array
    $array[0][0] = $array[count($array) - 1][0];
    $array[count($array) - 1][0] = $temp;
}

// Print the modified array
print_r($array);

?>

Array
(
    [0] => Array
        (
            [0] => g
            [1] => b
            [2] => c
        )

    [1] => Array
        (
            [0] => d
            [1] => e
            [2] => f
 ...

Time Complexity: O(1)

Auxiliary Space: O(1)