Description:
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The page at http://www.php.net/manual/en/function.shell-exec.php reads:

  Return Values
  The output from the executed command or NULL if an error occurred.

This is incorrect, as some of the commenters have noted. Successfully executed commands (exit status 0) that produce no output also return NULL. The return value is documented to be a string, so we would expect an empty string in the case of no output.

I would even argue that this is a bug, because it prevents detecting a non-zero exit status, but changing the behavior at this late stage would probably do more damage than good.

In any case, it needs to be properly documented.

Test script:
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$ php -r 'system("sync", $status); var_dump($status);'
int(0)

$ php -r '$rv = shell_exec("sync"); var_dump($rv);'
NULL

$ php -r '$rv = `sync`; var_dump($rv);'
NULL