Generate unique id or Index number in PHP

<?php
function setindex_prefix($index_assigned)
    {
        switch(strlen($index_assigned))
        {
            case 1:
                $new_index_assigned = "000".$index_assigned;
                break;
            case 2:
                $new_index_assigned = "00".$index_assigned;
            break;
			case 3:
                $new_index_assigned = "0".$index_assigned;
                break;
            default:
                $new_index_assigned = $index_assigned;
        }
        
    	return "NTRALMN".$new_index_assigned;
    }
	$unique_id=setindex_prefix($any_number);/* Pass parameter */
	echo $unique_id;
?>