Only variables should be passed by reference

As you know if you've fiddled with PHP's error_reporting setting, Drupal 7 is not yet E_STRICT compliant. E_STRICT compliance is a good idea for forward compatibility (PHP 5.3 and 6.0) and also helpful for servers that already log E_STRICT errors.

Many E_STRICT errors are evaluated at compile time, so E_STRICT error reporting should be enabled not in PHP code, but rather in the PHP configuration (e.g. php.ini file): error_reporting = E_ALL | E_STRICT

The most prevalent E_STRICT issue in core is numerous attempts to pass an expression (such as a function call), rather than a variable, by reference. This can be seen for example in the drupal_render($function()) pattern, e.g. drupal_render(drupal_get_form('some_form')). Running this code with E_STRICT error reporting results in: "Strict standards: Only variables should be passed by reference..."

The issue is that for functions like reset($array) or drupal_render($array), the array parameter is passed by reference because it is modified by the function. This means you must pass it a real variable and not a function (or expression) returning an array, because only actual variables can be passed by reference.

We can easily remedy this code snippet by creating an intermediate variable for the array:

A patch fixing these and other E_STRICT errors is being maintained at http://drupal.org/node/348448