PHP is a server language, a live chat system wouldn’t really be possible using only PHP as the page would have to refresh for every new comment in order for both parties to be able to see it so it would be a very clunky and unreliable system.
Languages required for a live chat system:
PHP & JAVASCRIPT
Libraries required for a live chat system:
JQUERY & AJAX (AJAX is built into JQUERY)
The following tutorial may be of interest.
https://www.webslesson.info/2018/07/live-chat-system-in-php-using-ajax-jquery.html
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solved Is it possible to make a chat using only php and the windows cmd? [closed]