Description:
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mb_strpos takes $needle, $haystack, $offset and $encoding as arguments but an error is thrown complaining of an empty delimiter when valid arguments are passed.

Test script:
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<?php
var_dump(mb_strpos("test", "test", 0, "UTF-8"));

Expected result:
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int(0)

Actual result:
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Warning: MB_strpos(): Empty delimiter in test.php on line 2