Description:
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Empty string in PHP doesn't equal U+0000 which is why mb_ord will not return 0 when passing empty string. On the other hand, ord will not complain about passing empty string. I know ord is not aware of any string encoding but this behavior however leads to a wrong assumption that empty string === U+0000 which in turn leads to bugs like https://bugs.php.net/bug.php?id=69162

So one might think if 

var_dump(ord('') === 0); // true

And

var_dump(ord("\u{0000}") === 0); // true

Then the following must be true

var_dump("\u{0000}" === ''); // false

Test script:
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var_dump(ord(''));

Expected result:
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Fatal error: Uncaught ValueError: ord(): Argument #1 ($str) must not be empty

Actual result:
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int(0)