Fix up PHP 8.3 tests

[dereuromark]

https://php.watch/versions/8.3/highlight_file-highlight_string-html-changes

[codecov-commenter]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (38a48dd) 94.25% compared to head (c35fa64) 93.82%.

❗ Current head c35fa64 differs from pull request most recent head b4233d9. Consider uploading reports for the commit b4233d9 to get more accurate results

Files	Patch %	Lines
src/Highlighter/PhpHighlighter.php	75.00%	1 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff              @@
##             master      #10      +/-   ##
============================================
- Coverage     94.25%   93.82%   -0.44%     
- Complexity       56       58       +2     
============================================
  Files             9        9              
  Lines           174      178       +4     
============================================
+ Hits            164      167       +3     
- Misses           10       11       +1     

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