Can't use function return value in write context?

In PHP, this error occurs when a function's return value is used in a context that expects a variable (a "write context"). This commonly happens with language constructs like isset(), unset(), or when attempting reference assignment. (Note: empty() was updated in PHP 5.5 to accept expressions, but the error remains common for the other constructs.)

For example, this will trigger the error:

isset(trim($str));

The error happens because these constructs require a variable, not a function result. To fix it, assign the function's return value to a variable first, then pass that variable to the construct:

$trimmed = trim($str);
isset($trimmed);

This also applies to unset() and reference assignment:

// Incorrect
unset(trim($str));
$ref = &trim($str);

// Correct
$value = trim($str);
unset($value);
$ref = &$value;