Description:
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When using highlight_string(), it will often not highlight the 
string I pass it.

Reproduce code:
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<?php echo highlight_string('<test>', true); ?>

Expected result:
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The code should be highlighted, but sometimes isn't.

Actual result:
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Sometimes, the code remains unhighlighted, but appears to have 
htmlspecialchars run on it. This seems to vary wildly, and it 
can be extremely difficult for it to highlight at all. Other 
times, it will highlight every time, for hours.