WordPress Codex: wp_set_post_tags()

Set tags for a post. Every tag that does not already exists will be automatically created. Uses wp_set_post_terms().

Usage
wp_set_post_tags( $post_ID, $tags, $append )
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Parameters
$post_ID
(integer) (required) Post ID.
Default: 0

$append
(boolean) (optional) If true, tags will be appended
to the post. If false, tags will replace existing tags.
Default: false

Return values
(boolean|null)
Will return false if $post_id is not an
integer or is 0. Will return null otherwise.

Examples
To add the tags meaning and life to the current tags of the post with ID 42:
wp_set_post_tags( 42, 'meaning,life', true );

More informations: http://codex.wordpress.org/Function_Reference/wp_set_post_tags