How to find out if the year is leap year?

Another way is to use the date function with the L parameter that returns 1 is in leap year, 0 otherwise.

Example for current year:

echo date('L');

For a specific year:

$ano = 2007;
$bissexto= date('L', mktime(0, 0, 0, 1, 1, $ano));
echo $ano . ' ' . ($bissexto? 'é' : 'não é') . ' um ano bissexto.';

See working at ideone .

Another way to know if the year is leap is to know the amount of days in February, this can be done with the argument t of the function date()

 echo  date('t', strtotime('2016-02-01')); //29

One workaround is to use a native PHP function that is suitable for this, cal_days_in_month

Examples:

<?php
//2015 não é bisexto
$result = cal_days_in_month(CAL_GREGORIAN, 2, 2015) === 29;
var_dump($result);

//2016 é bisexto
$result = cal_days_in_month(CAL_GREGORIAN, 2, 2016) === 29;
var_dump($result);

Example online: link

A simple function:

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

Using:

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

Online: link

To do such an operation you can use two ways, but the two lead to the same method: Verify that February ends with the 29th.

Strtotime and date

(date('d', strtotime('last day of february this year')) === '29')

DateTime Object

(new DateTime('last day of february this year'))->format('d') === '29')

This solution is so simple, that some people questioned me in the chat if I was joking. To be convincing, I made an example in IDEONE:

link

You can use this syntax for any year. If you want to check another year, you could change the excerpt this year to 2018 or next year for example.