This question already has an answer here:
<?php
mysql_connect("localhost","root","");
mysql_select_db("univ_management");
?>
this is the following block of the code
Uncaught Error: Call to undefined function mysql_connect() in C:\xampp\htdocs\ums4\db_con.php:2 Stack trace: #0 C:\xampp\htdocs\ums4\action.php(10): include_once() #1 {main} thrown in C:\xampp\htdocs\ums4\db_con.php on line 2
</div>
First and foremost: do not use mysql_* functions. Just don't, there is no excuse. These have long since been deprecated (with good reason). Instead, use the mysqli_* functions (note the i) or the PDO library. With that out of the way, an actual answer to your question:
Though dense, the error message describes exactly the problem:
... Call to undefined function mysql_connect() ...
This implies that PHP is missing the appropriate shared library that might define that function. Given the currently supported versions of PHP, I suspect you are using PHP7+. This is important because the mysql_* functions were removed in PHP v7.0.0.