<?php

$data = '<meta name="keywords" content="keyword1,keyword2,keyword3" />
<meta name="keywords" content="keyword1,keyword2,keyword3">
<META NAME="keywords" CONTENT="keyword1,keyword2,keyword3">
<META NAME="keywords" CONTENT="keyword1,keyword2,keyword3" />
<META name="keywords" CONTENT="keyword1,keyword2,keyword3" >';


preg_match_all('#="keywords" content="(.*?)"#si',$data,$veri);

echo '<pre>';
print_r($veri[1]);
?>

Array
(
    [0] => keyword1,keyword2,keyword3
    [1] => keyword1,keyword2,keyword3
    [2] => keyword1,keyword2,keyword3
    [3] => keyword1,keyword2,keyword3
    [4] => keyword1,keyword2,keyword3
)