Using one equals sign in an if statement

Question

In a PHP if statement, when I am comparing a variable I would need to use two == signs. For example,

if($foo == 'bar'){
    //Run code
}

But if I were to use one equals sign like this:

if($foo = 'bar'){
    //Run code
}

This would actually set the $foo variable.

Why would you ever need to set a variable like this inside an if statement? I can't think of one instance where this would be useful, and many programmers I've spoken to feel the same way.

Is there a reason why PHP allows you to set a variable this way?

Answer 1

Testing the result of a function that returns an error:

if (($err = some_function())) {
    echo "Error: " . $err;
}

Answer 2

PHP, like also C, allows to write any expression into an if-condition.

This can be an expression like:

if(!$db=new database()) die("NO DATABASE CONNECTION");

But it has rather the background that the expression $a="abc" is evaluated a different way:

1. Set the value of $a to abc
2. Rest of the expression is: if($a)
3. $a is != 0, so it is true MOST IMPORTANT
4. Expression is true, go into if-case

So the expression is evaluated to if(true) because the priority table of operators make the expression succeed. It is not a mistake of PHP, it is rather the definition of TRUE/FALSE and how things get evaluated.

This can also be used in loops like shown in another answer for the loop expression with $db->fetch()

Answer 3

One of the most common usecases is to make sure something exists (or is truthy), and then use it inside the block, for example:

while ($row = $dbConnection->fetch()) { //Make sure that $dbConnection->fetch() is not false or NULL...
    //You can use $row here!
}