How to calculate age from a birth date

Short answer

The diff() method returns the interval between dates; the y property is the number of full years:

<?php
 
$birth = new DateTimeImmutable('1990-06-15');
$today = new DateTimeImmutable('2025-01-30');
 
echo $birth->diff($today)->y;

Output:

Checking adulthood

Compare the number of years with the threshold:

<?php
 
$birth = new DateTimeImmutable('2010-03-01');
$today = new DateTimeImmutable('2025-01-30');
 
var_dump($birth->diff($today)->y >= 18);

Output:

bool(false)

Functions used in this recipe

DateTimeImmutable — A date and time object that cannot be changed: every operation returns a new object.
checkdate — Checks whether such a date exists.