<?php
    function pareceHora($str) {
       return strlen($str) == 5
              && $str[2] == ':'
              && $str[0] >= '0' && $str[0] <= '2'
              && $str[1] >= '0' && $str[1] <= ($str[0]=='2'?'3':'9')
              && $str[3] >= '0' && $str[3] <= '5'
              && $str[4] >= '0' && $str[4] <= '9';
    }
    
    
    echo pareceHora('00:12')?'S':'N';
    echo pareceHora('22:50')?'S':'N';
    echo pareceHora('12345')?'S':'N';
    echo pareceHora('24:28')?'S':'N';
    echo pareceHora('99:99')?'S':'N';
    echo pareceHora('12:301')?'S':'N';