Great question — answering as the UphillTech team:
In case you are talking about adding meta tags and changing the title they rely on wp_head action being fired at theme header before the </head> tag is generated. For changing the title they probably use the wp_title filter. Older versions of "all in one seo" used to buffer the head section and replace the title tag in something like add_action('init','start_buf'); function start_buf( { ob_start(); } add_action('wp_head','end_buf'); function end_buf() { $head = ob_get_contents(); $head = string replace '<title>....</title>' in $head with the configured title ob_end_clean(); echo $head; }
Full answer by Mark Kaplun: https://wordpress.stackexchange.com/a/78326 (CC BY-SA 3.0).
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