Why does PHP consider 0 to be equal to a string?

Question

I have the following piece of code:

$item[\'price\'] = 0;
/* Code to get item information goes in here */
if($item[\'price\'] == \'e\') {
    $item[\'price\'         


        

Answer 1

Your problem is the double equal operator, which will typecast the right member to the type of the left. Use strict if you prefer.

if($item['price'] == 'e') {
    $item['price'] = -1;
}

Let's go back to your code (copied above). In this case, in most cases, $item['price'] is an integer (except when it is equal to e, obviously). As such, by laws of PHP, PHP will typecast "e" to integer, which yields int(0). (Don't believe me? <?php $i="e"; echo (int)$i; ?>).

To easily get away from this, use the triple equal (exact comparison) operator, which will check the type and will not implicitly typecast.

P.S: a PHP fun fact: a == b does not imply that b == a. Take your example and reverse it: if ("e" == $item['price']) will never actually be fulfilled provided that $item['price'] is always an integer.

Answer 2

You are doing == which sorts out the types for you.

0 is an int, so in this case it is going to cast 'e' to an int. Which is not parsable as one and will become 0. A string '0e' would become 0 and would match!

Use ===

Answer 3

"ABC" == 0

evaluates true because first "ABC" is converted to integer and becomes 0 then it is compared to 0.

This is an odd behaviour of the PHP language: normally one would expect 0 to be promoted to string "0" and then compared to "ABC" with a result false. Perhaps that's what happen in other languages like JavaScript where the weak comparison "ABC" == 0 evaluates false.

Doing a strict comparison solves the problem:

"ABC" === 0

evaluates false.

But what if I do need to compare numbers as strings with numbers?

"123" === 123

evaluates false because the left and right term are of different type.

What is actually needed is a weak comparison without the pitfalls of PHP type juggling.

The solution is to explicit promote the terms to string and then do a comparison (strict or weak doesn't matter anymore).

(string)"123" === (string)123

is

true

while

(string)"123" === (string)0

is

false

---

Applied to the original code:

$item['price'] = 0;
/*code to get item information goes in here*/
if((string)$item['price'] == 'e') {
    $item['price'] = -1;
}