<?php if (is_single()) {
foreach((get_the_tags()) as $tag) {
$keywords[] = strtolower($tag->name);
}
foreach((get_the_category()) as $category) {
$keywords[] = strtolower($category->cat_name);
}
?>
<meta name="keywords" content="keyword 1, keyword 2, <?php echo implode(", ", array_unique($keywords)); ?>" />
<?php }
else { ?>
<meta name="keywords" content="keyword 1, keyword 2,Keyword3" />
<?php } ?>

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