$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('#<meta\s+name\s*=\s*["\'](.*?)["\']\s+content\s*=\s*["\'](.*?)["\'][/\s]*>#si', $data, $cikti);

echo '<pre>';
print_r($cikti);

Array
(
    [0] => Array
        (
            [0] => 
            [1] => 
            [2] => 
            [3] => 
            [4] => 
        )

    [1] => Array
        (
            [0] => keywords
            [1] => keywords
            [2] => keywords
            [3] => keywords
            [4] => keywords
        )

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

)