fix: replace deprecated mysql_* with mysqli_* in upgrade scripts

[Frankli9986]

Problem

Fixes #572

When installing OpenCATS with demo content on PHP 7+, the installation fails because database upgrade scripts still use the deprecated mysql_* functions, which were removed in PHP 7.0.

Changes

• mysql_real_escape_string() → mysqli_real_escape_string() (2 occurrences)
• mysql_fetch_row() → mysqli_fetch_row() (1 occurrence)
• Added global \; where needed to ensure the mysqli connection resource is available in the upgrade script context.

Files Modified

• modules/install/Schema.php

Testing

• Verified that no other mysql_* function calls remain in the codebase (only these 3 were found in upgrade script strings).
• The fix aligns with the existing mysqli_* usage already present in modules/install/ajax/ui.php.

Related Issue

• Demo content doesn't work #572

[anonymoususer72041]

Hi @Frankli9986, thanks for looking into this. This looks closely related to and partly overlapping with my existing PR #765.

From what I can see, this PR fixes the same obsolete mysql_* calls in modules/install/Schema.php that are already covered in #765. The main difference is that #771 replaces them with direct mysqli_* calls, while #765 routes them through the existing DatabaseConnection API where possible. In particular, #765 also touches the same install schema areas and includes a broader cleanup of legacy database result handling around install, backup and control panel code.

I think it would be better to consolidate the work in one PR rather than having two separate fixes for the same issue. You are very welcome to join #765 and submit suggestions there, especially because there are small differences between both approaches that may still be useful to discuss or incorporate.

I also started a discussion in the comments of #765 about a few open questions before merging, including whether changing previous migrations is the right approach. I would be happy if you joined that discussion as well.