$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;
}
}