Why do some people put the value before variable in if statement?

Short answer

Some people do it in order to avoid mistakenly using the assignment operator (=) when they really meant to use a comparison operator (== or ===).

Long answer

In PHP there are 3 operators that can be mistaken for eachother:

• = is the assignment operator
• == is the "equal to" operator
• === is the "identical to" operator

The first operator is only used for assigning a value to a variable. The second and third are only used for comparing two values, or expressions, against eachother.

In PHP it is possible to assign a value to a variable inside control structures (if, for, while, etc.). This may cause problems if you are not careful. For example, if you want to compare $a against a boolean value you can do it like this:

if ($a == false) // or
if ($a === false)

If you are not careful, however, it may end up like this:

if ($a = false)

... which is perfectly legal code, but it is an assignment and will eventually cause problems. The reason it will cause problems is because the expression, $a=false, will be evaluated and the code will keep running as if nothing is wrong, when in fact it was not intended.

The reason some people switch around the operands, when comparing a value against a literal (fixed value, like true, false, 7, 5.2, and so on), is because it is not possible to assign a value to a literal. For example:

if (false = $a)

... will cause an error, telling the programmer that they made a mistake, which they can then fix.

They are equivalent.

Some programmers prefer this "Yoda style" in order to avoid the risk of accidentally writing:

if ($variable = true) {
    // ...
}

, which is equivalent to

$variable = true;
// ...

, when they meant to write

if ($variable === true) {
    // ...
}

(whereas if (true = $variable) would generate an obvious error rather than a potentially-subtle bug).

I guess this is just the way of thinking in general. When you really think about a deep if statement, you think if that statement is true, not the other point of view. It's not wrong, but in my head naming it inverse, would annoy me and make me lose concentration about the statement. So I would say it's just the way people think :D.