Description:
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If strtr(string $str, array $replace_pairs) is called and one of the keys of $replace_pairs is an empty string, strtr will return the boolean value FALSE.  This is not reflected in the documentation.

Test script:
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<?php
var_dump(strtr('foo', array('' => 'nothing')));

Expected result:
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Unsure, is the empty string ever really "found" in a string?

Actual result:
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bool(false)