Description:
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mb_substr takes an int argument for length, but does not accept all integers on a 64-bit compiled PHP. Instead, the number is treated as -(1<<31), and the string is therefore truncated to zero length.

Test script:
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<?php
assert('"1" == mb_substr("1", 0, PHP_INT_MAX)');

Expected result:
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No output

Actual result:
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PHP Warning:  assert(): Assertion ""1" == mb_substr("1", 0, PHP_INT_MAX)" failed in t.php on line 2
PHP Stack trace:
PHP   1. {main}() t.php:0
PHP   2. assert() t.php:2