[Docs] Fix example php code in docs.

[sfadschm]

This is the follow up of #5689.

It aims to make all php examples executable and apply our CS rules.

Status:

• Convert non-php code.
• Display opening tags (for PHP) and closing tags (for mixed HTML).
• Remove occurences of . :.
• Replace utf-8 characters (except the euro and pound signs).
• Fix missing class definitions.
• Fix remaining syntax errors.
• Enable and run php-cs-fixer for user_guide_src\source.
• Resolve merge conflicts.

[kenjis]

@sfadschm
This PR is going to be very big.
Can you break it up into smaller PRs?

[sfadschm]

@sfadschm This PR is going to be very big. Can you break it up into smaller PRs?

Yes, will do 😄

[sfadschm]

I am splitting this up and will rebase the later ones when the first ones are merged.