Description:
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Neither the documentation for $start nor the return values section mention that the function may return FALSE.

The documentation for $length does mention returning FALSE. Since $length is an optional parameter and doesn't really affect whether or not the function returns FALSE, it is not an intuitive place for the documentation

I suggest:

1.) Moving the following line from the $length parameter documentation to the $start parameter documentation:

"If $string is less than or equal to $start characters long, FALSE  will be returned."

2.) Altering the return documentation to read:

"Returns the extracted part of string. If $string is less than or equal to $start characters long, FALSE will be returned."

See also http://bugs.php.net/bug.php?id=20695&edit=2