Comparison order in PHP IF

Question

I'm pretty sure that there is no difference between this

if ($value === true)

and this

if (true === $value)

For me it always seems confusing. I used to think that it's a 'bad unconventional style of new developers'. Furthermore, my boss told me never to do such a thing.

But today I was looking through Slim's source code, written by the guy who created PHP: The Right Way, and saw this (line 341).

if (true === $value) {
    $c['settings'] = array_merge_recursive($c['settings'], $name);
}

I'm sure a guy like Josh Lockhart wouldn't do something like this if it was considered a bad practice. So what's with this order? Is it some kind of an old tradition?

Answer 1

This is colloquially referred to as 'Yoda Conditions'. The reasoning behind their usage is because sometimes people make mistakes. In the context of an if statement, this is one such mistake that is sometimes made:

if($value = 42)

Notice there is no double equals sign (==). This is a valid statement and is not a syntax error. It is possible that it would cause huge disruptions with the code that follows it, and it is very hard to identify quickly. Yoda conditions get around this by reversing the conditional expression:

if(42 = $value)

This is not a valid statement and is a syntax error, and those are generally reported with a line number included, so they're pretty easy to find.

_{"Syntax error you have. Line 73 do something else you must." -- Yoda's compiler, probably}

Whether or not you actually think they're good or bad is up to personal preference and style. In some cases of popular frameworks (such as Wordpress), Yoda conditions are actually part of the official coding standards. The major critique against them is that, to some, they decrease readability of the code without providing any real major benefit.

As @MonkeyZeus pointed out in a comment to the question, sometimes people do actually mean to only use a single equals sign inside of an if condition:

if($iMustHaveAValue = functionCallThatCanFail()) {
    // Rest assured, $iMustHaveAValue has a truthy value.
}

Since the value of an assignment operation is the value assigned, the value that is evaluated by the if condition in this case (and in the original case which was unintended) is whatever value happened to be assigned.