How can I rotate a 2d array in php by 90 degrees

Another reliable option:

function rotateMatrix90( $matrix )
{
    $matrix = array_values( $matrix );
    $matrix90 = array();

    // make each new row = reversed old column
    foreach( array_keys( $matrix[0] ) as $column ){
        $matrix90[] = array_reverse( array_column( $matrix, $column ) );
    }

    return $matrix90;
}

Less clever than @mark-baker's by far. Maybe more clear.

I showed you how to transpose an array in answer to a previous question, to rotate 90 degrees, use that transpose logic, and then reverse the order of the values in each row in turn:

$matrix = [
    [1, 2, 3],
    [4, 5, 6], 
    [7, 8, 9],
];

array_unshift($matrix, null);
$matrix = call_user_func_array('array_map', $matrix);
$matrix = array_map('array_reverse', $matrix);
var_dump($matrix);

Demo

php doesn't have concepts like "transpose" for a matrix without adding some sort of linear algebra library. you can do it natively by eaching through the matrix and swapping some indexes

<?php

function rotate90($mat) {
    $height = count($mat);
    $width = count($mat[0]);
    $mat90 = array();

    for ($i = 0; $i < $width; $i++) {
        for ($j = 0; $j < $height; $j++) {
            $mat90[$height - $i - 1][$j] = $mat[$height - $j - 1][$i];
        }
    }

    return $mat90;
}

$mat = [[1, 2, 3], [4, 5, 6], [7, 8, 9]];
print_r($mat);
//123
//456
//789
print_r(rotate90($mat));
//741
//852
//963


$mat = [[1, 2, 3], [4, 5, 6], [7, 8, 9], ["a", "b", "c"]];
print_r($mat);
//123
//456
//789
//abc
print_r(rotate90($mat));
//a741
//b852
//c963