Obtenha anos entre duas datas usando DateTime

Usando a classe DateTime do PHP:

// My Birthday :)
$date_1 = new DateTime( '1989-06-15' );

// Todays date
$date_2 = new DateTime( date( 'Y-m-d' ) );

$difference = $date_2->diff( $date_1 );

// Echo the as string to display in browser for testing
echo (string)$difference->y;


/**
 * In a function
 */
function yearsMonthsBetween ( $date1, $date2 ) {

    $d1 = new DateTime( $date1 );
    $d2 = new DateTime( $date2 );

    $diff = $d2->diff( $d1 );

    // Return array years and months
    return array ( 'years' => $diff->y, 'months' => $diff->m );
}