Can't use method return value in write context

Note: This is a very high voted answer with a high visibility, but please note that it promotes bad, unnecessary coding practices! See @Kornel's answer for the correct way.

Note #2: I endorse the suggestions to use @Kornel's answer. When I wrote this answer three years ago, I merely meant to explain the nature of the error, not necessarily endorse the alternative. The code snippet below is not recommended.

It's a limitation of empty() in PHP versions below 5.5.

Note: empty() only checks variables as anything else will result in a parse error. In other words, the following will not work: empty(trim($name)).

You'd have to change to this

// Not recommended, just illustrates the issue
$err = $r->getError();
if (!empty($err))

Prior to PHP 5.5, the the PHP docs used to say:

empty() only checks variables as anything else will result in a parse error

In PHP < 5.5 you weren't able use empty() directly on a function's return value. Instead, you could assign the return from getError() to a variable and run empty() on the variable.

In PHP >= 5.5 this is no longer necessary.

I usually create a global function called is_empty() just to get around this issue

function is_empty($var)
{ 
 return empty($var);
}

Then anywhere I would normally have used empty() I just use is_empty()

As pointed out by others, it's a (weird) limitation of empty().

For most purproses, doing this is equal as calling empty, but this works:

if ($r->getError() != '')

I'm not sure if this would be a common mistake, but if you do something like:

$var = 'value' .= 'value2';

this will also produce the same error

Can't use method return value in write context

You can't have an = and a .= in the same statement. You can use one or the other, but not both.

Note, I understand this is unrelated to the actual code in the question, however this question is the top result when searching for the error message, so I wanted to post it here for completeness.