gistfile1.php

<?php

class YmLinerTest
{
    public function test(array $array)
    {
        sort($array);//一応ソート
        $date = new DateTime($array[0].'01');
        foreach ($array as $ym) {
            if ($date->format('Ym') != $ym) return false;
            $date->modify('+1 month');
        }
        return true;
    }
}

$tester = new YmLinerTest();
var_dump($tester->test(array("201301", "201302", "201303")));
var_dump($tester->test(array("201301", "201302", "201304")));

続けるにはENTERを押すかコマンドを入力してください
bool(true)
bool(false)