Description:
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Please, add something like:
For non-strings trim() can return null

( E.g. http://www.php.net/manual/en/function.parse-url.php has a similar remark )

Test script:
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<?
$a = new \stdClass();
$b = trim( $a );

var_dump($b);

Expected result:
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Warning: trim() expects parameter 1 to be string, object given in ... on line 3
string(0) ""

Actual result:
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Warning: trim() expects parameter 1 to be string, object given in ... on line 3
NULL