fix: Update form_helper.php

[virginus01]

Description

This PR enhances the form_open function in the following ways:

1. Function Existence Check:

   • Added a check to ensure the function form_open is defined only once to avoid redeclaration errors.

2. Documentation:

   • Included a detailed docblock to describe the function, its parameters, and their types for better understanding and maintenance.

3. Attribute Consistency:

   • Updated the method attribute to uppercase "POST" and adjusted the check for the 'GET' method to uppercase "GET" to maintain consistency.

4. CSRF and Charset Handling:

   • Ensured that CSRF and accept-charset attributes are handled consistently.

The updates to the form_open function make it work better under different conditions and make the code easier to read.

Description Explain what you have changed, and why.

Checklist:

• [x] Securely signed commits
• [ ] Component(s) with PHPDoc blocks, only if necessary or adds value
• [ ] Unit testing, with >80% coverage
• [ ] User guide updated
• [ ] Conforms to style guide