function get_age($birthday)
{
    $birth = strtotime($birthday);
    $now = strtotime('now');

    $age =  ($now - $birth) / 31536000;

    echo floor($age);
}

get_age('15 march 1984');

This function will get the age in years. It compares the inserted date with the current date and outputs the result.

Get age in years

PHP