Add additional PHP LFI functions: file_get_contents, readfile, and fopen

[matejsmycka]

Link to an issue, if relevant

Issue link

Adding a new rule? Look over this PR checklist

• The issue or PR has links, references, or examples.
• The rule has true positive and true negative test cases in a file that matches the rule name.

If the rule is my-rule, the test file name should be my-rule.js.

True positives are marked by comments with ruleid: <my-rule> and true negatives are marked by comments with ok: <my-rule>.

• The rule has a good message. A good message includes:

1. A description of the pattern (e.g., missing parameter, dangerous flag, out-of-order function calls).
2. A description of why this pattern was detected (e.g., logic bug, introduces a security vulnerability, bad practice).
3. An alternative that resolves the issue (e.g., use another function, validate data first, discard the dangerous flag).

[CLAassistant]

All committers have signed the CLA.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

Matej Smycka seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}