Description:
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According to the documentation of the version_compare() function, it returns either true or false, if there is an operator parameter given.

https://www.php.net/manual/en/function.version-compare.php

But it seems it can also return null, if there is an invalid operator string given.
Invalid can be at least either a totally wrong operator, e.g. "invalid", a space " " or a valid operator string pre- or suffixed with a space, e.g " ge".

This is certainly a documentation issue but for the future this could be improved by throwing an exception, e.g. InvalidArgumentException.

By the way this was reported as a comment on the function's documentation page at https://www.php.net/manual/en/function.version-compare.php#106715

Test script:
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<?php
var_dump(version_compare('3.7.5', '3.7.0', 'ge')); // true, OK
var_dump(version_compare('3.7.5', '3.7.0', ' ')); // null, should throw invalid argument exception at best
var_dump(version_compare('3.7.5', '3.7.0', ' ge')); // null, should throw invalid argument exception at best
var_dump(version_compare('3.7.5', '3.7.0', 'ge ')); // null, should throw invalid argument exception at best
var_dump(version_compare('3.7.5', '3.7.0', 'invalid')); // null, should throw invalid argument exception at best
?>

https://3v4l.org/CrO0J

Expected result:
----------------
That the possible return value of null is officially documented at https://www.php.net/manual/en/function.version-compare.php.
Even better would be an exception thrown, e.g. InvalidArgumentException.

Actual result:
--------------
null (officially undocumented)