<?php

function valid_time($time){
        $m = substr($time, -2);
        $h = (explode(':', substr($time, 0, -2)));
        $h = $h[0];
        return (is_numeric($h) && is_numeric($m) && $h>0 && $h<13 && $m>=0 && $m<60);
}

$times = array(
       '100',          # valid
       '1100',         # valid
       '1300',         # invalid
       '01:00',        # valid
       '12:59',        # valid
       '00:01',        # invalid
       '00:25pm',      # invalid
       '0000',         # invalid
       'a00',          # invalid
       '00',           # invalid
       );
foreach($times as $time){
        printf("%10s => %7s\n",  $time,  (valid_time($time)? "valid" : "invalid"));
}
?>