Uses PHP's built in SPL Array Iterators to quickly spin through a multidimensional array and return a 'flattened' output.

flattenArray.php

<?php

function flattenArray( array $array )
{
    $returned_array = array();
    
    foreach( new \RecursiveIteratorIterator( new \RecursiveArrayIterator($array) ) as $key => $value  )
    {
        $returned_array[] = $value;
    }

    return $returned_array;
}