<?php 

function birthday($birthday){ 
$age = strtotime($birthday); 
if($age === false){ 
 return false; 
} 
list($y1,$m1,$d1) = explode("-",date("Y-m-d",$age)); 
$now = strtotime("now"); 
list($y2,$m2,$d2) = explode("-",date("Y-m-d",$now)); 
$age = $y2 - $y1; 
if((int)($m2.$d2) < (int)($m1.$d1)) 
 $age -= 1; 
return $age; 
} 
echo birthday('1986-07-22'); 
?>

<?php
echo birthday("1989-01-25");
function birthday2($birthday){
 list($year,$month,$day) = explode("-",$birthday);
 $year_diff = date("Y") - $year;
 $month_diff = date("m") - $month;
 $day_diff  = date("d") - $day;
 if ($day_diff < 0 || $month_diff < 0)
  $year_diff--;
 return $year_diff;
}