Undefined index Error While Using FORM Tag in .php File [duplicate]

If I Use forms inside of a .html File, It Works Properly But When I Use it inside a .php file, Its Not Working Properly.

HTML (pr.php): Code Image

PHP Code (send.php): PHP Code Image

Error : Notice: Undefined index: chat-input in /storage/ssd3/2-8/18--9228/public_html/send.php on line 9

source https://stackoverflow.com/questions/71327191/undefined-index-error-while-using-form-tag-in-php-file