Group data from a 3-level array data by a second level column and merge the third level data per group

I want to group the following data by the first column and merge the other column's subarray data per group.

$arr = [
    [0, [0 => 'abc']],
    [0, [1 => 'def']],
    [1, [0 => 'ghi']],
];

Desired result:

Array
    (
       [0] => Array
           (
              [0] => 'abc'
              [1] => 'def'
            )
            
       [1] => Array
           (
               [0] => 'ghi'
            )
                
    )

I'm not 100% sure I've understood what you want, but if I have, this should work:

<?php

$arr = Array(
    0 => Array(0, Array(0=>'abc')),
    1 => Array(0, Array(1=>'def')),
    2 => Array(1, Array(0=>'ghi'))
);


function transformArray($array) {
    $newArray = array();

    foreach ($array as $value) {
        if (!isset($newArray[$value[0]])) {
            $newArray[$value[0]] = array();
        }
        $newArray[$value[0]][] = array_pop($value[1]);
    }

    return $newArray;
}


$outputArray = transformArray($arr);
echo '<pre>' . print_r($outputArray, true) . '</pre>';

?>