Why should htmlentities be applied only during output and not on post variables?

When htmlentities are applied to post variables, it can interfere with data processing and storage, potentially causing unintended behavior or data corruption. It is best practice to apply htmlentities only during output to ensure that the data is displayed safely without altering its original form. This helps prevent security vulnerabilities such as cross-site scripting (XSS) attacks.

// Applying htmlentities during output
echo htmlentities($_POST['input']);