What are some common issues with variable passing between .htm and .php files in PHP?

Common issues with variable passing between .htm and .php files in PHP include not properly using the $_GET or $_POST superglobals to retrieve the variables passed from the .htm file, or not properly encoding or decoding the variables when passing them between the files. To solve this issue, make sure to use $_GET or $_POST superglobals to retrieve the variables passed from the .htm file, and encode or decode the variables using functions like urlencode() and urldecode() when passing them between the files.

// .htm file
<a href="example.php?var=<?php echo urlencode($var); ?>">Link</a>

// example.php file
$var = urldecode($_GET['var']);