具体如下：

/**
 * 根据出生年月日计算出年龄
 * @param $birth_year 
 * @param $birth_month
 * @param $birth_day
 * @return int
 */
function getAgeByBirth($birth_year,$birth_month,$birth_day){
  if(empty($birth_year) || empty($birth_month) || empty($birth_day)){
    return 0;
  }
  $current_year = date('Y',time());
  $current_month = date('m',time());
  $current_day = date('d',time());
  if($birth_year >= $current_year){
    return 0;
  }
  $age = $current_year - $birth_year - 1;
  if($current_month>$birth_month){
    return $age+1;
  }else if($current_month == $birth_month && $current_day>=$birth_day){
    return $age+1;
  }else{
    return $age;
  }
}
//测试：
echo getAgeByBirth('1988','8','8');

运行结果：

31