Check for leap years between 2 dates.

leapyear_one.php

<?php 

$startYear = 1990;
$endYear = 2017;

$yearsToCheck = range($startYear, $endYear);

foreach ($yearsToCheck as $year) {
    $isLeapYear = (bool) date('L', strtotime("$year-01-01"));
    if($isLeapYear === true) echo $year . " is a leap year <br>";
}

/*
1992is a leap year 
1996is a leap year 
2000is a leap year 
2004is a leap year 
2008is a leap year 
2012is a leap year 
2016is a leap year 
*/

leapyear_three.php

<?php 

function findLeapYears()
{
	if($_SERVER['REQUEST_METHOD'] === 'POST') 
	{
		$startYear = $_POST['start-year'];
		$endYear = $_POST['end-year'];
		$result = [];

		$yearsToCheck = range($startYear, $endYear);

		if(!empty($startYear) && !empty($endYear))
		{
			foreach ($yearsToCheck as $year) {
			    $isLeapYear = (bool) date('L', strtotime("$year-01-01"));
			    if($isLeapYear === true) $result[] .= $year . "is a leap year";
			}
		}

		return $result;
	}
}
?>

<form method="post">
	<input type="text" name="start-year">
	<input type="text" name="end-year">
	<input type="submit" name="submit">
</form>

<?php var_dump(findLeapYears()) ?>

<!-- 
array (size=7)
  0 => string '1992is a leap year' (length=18)
  1 => string '1996is a leap year' (length=18)
  2 => string '2000is a leap year' (length=18)
  3 => string '2004is a leap year' (length=18)
  4 => string '2008is a leap year' (length=18)
  5 => string '2012is a leap year' (length=18)
  6 => string '2016is a leap year' (length=18)
-->

leapyear_two.php

<?php

$startYear = 1990;
$endYear = 2017;

$yearsToCheck = range($startYear, $endYear);

foreach ($yearsToCheck as $year) {
    $isLeapYear = (bool) date('L', strtotime("$year-01-01"));

    printf(
        '%d %s a leap year %s',
        $year,
        $isLeapYear ? 'is' : 'is not',
        "<br>"
    );
}
/*
1990 is not a leap year 
1991 is not a leap year 
1992 is a leap year 
1993 is not a leap year 
1994 is not a leap year 
1995 is not a leap year 
1996 is a leap year 
1997 is not a leap year 
1998 is not a leap year 
1999 is not a leap year 
2000 is a leap year 
2001 is not a leap year 
2002 is not a leap year 
2003 is not a leap year 
2004 is a leap year 
2005 is not a leap year 
2006 is not a leap year 
2007 is not a leap year 
2008 is a leap year 
2009 is not a leap year 
2010 is not a leap year 
2011 is not a leap year 
2012 is a leap year 
2013 is not a leap year 
2014 is not a leap year 
2015 is not a leap year 
2016 is a leap year 
2017 is not a leap year 
*/