<?php
    function isLeapYear($year = NULL) {
         $year = is_numeric($year) ? $year : date('Y');
         return cal_days_in_month(CAL_GREGORIAN, 2, $year) === 29;
    }

    var_dump(isLeapYear());//Ano atual
    var_dump(isLeapYear(2015));//Ano 2015
    var_dump(isLeapYear(2016));//Ano 2016
    var_dump(isLeapYear(2017));//Ano 2017