This PHP function accepts a birthday as a parameter and will return the age based on that birthday. Handy for social sites/applications.

gistfile1.php

<?php 
// PHP 5.3-
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('1981-05-18'); 

// PHP 5.3+
function birthday($birthday) {
    $age = date_create($birthday)->diff(date_create('today'))->y;
    
    return $age;
}

echo birthday('1981-05-18'); 

If php >= 5.3 you can reduce this to just 1 line by using:

echo date_create("1981-05-18")->diff(date_create("today"))->y;

@kisuka Gist updated. Thanks!