What are common server-side restrictions that may cause a "Forbidden" error when submitting forms in PHP?

When submitting forms in PHP, a "Forbidden" error can occur due to server-side restrictions such as incorrect file permissions, mod_security rules, or server configuration settings blocking the form submission. To resolve this issue, check the file permissions of the PHP script handling the form submission, ensure that mod_security rules are not blocking the request, and review server configuration settings to allow form submissions.

// Example code to set correct file permissions for the PHP script handling form submission
chmod('submit_form.php', 0644);