convert multidimensional array to single array php

$arrayMult = [ ['a','b'] , ['c', 'd'] ];
$arraySingle = call_user_func_array('array_merge', $arrayMult);
// $arraySingle is now = ['a','b', 'c', 'd'];

$singleArray = []; 
foreach ($parentArray as $childArray) 
{ 
    foreach ($childArray as $value) 
    { 
    $singleArray[] = $value; 
    } 
}