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

I have the following piece of code:

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

It is intended to initialize the item price to 0 and then get information about it. If the price is informed as 'e' it means an exchange instead of a sell, which is stored in a database as a negative number.

There is also the possibility to leave the price as 0, either because the item is a bonus or because the price will be set in a later moment.

But, whenever the price is not set, which leaves it with the initial value of 0, the if loop indicated above evaluates as true and the price is set to -1. That is, it considers 0 as equal to 'e'.

How can this be explained?

When the price is provided as 0 (after initialization), the behavior is erratic: sometimes the if evaluates as true, sometimes it evaluates as false.*

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 ===

From PHP.net:

> Comparisons between strings and numbers using == and other non-strict > comparison operators currently work by casting the string to a number, > and subsequently performing a comparison on integers or floats. This > results in many surprising comparison results, the most notable of > which is that 0 == "foobar" returns true.

However this behavior was changed in PHP 8.0:

> When comparing to a numeric string, PHP 8 uses a number comparison. > Otherwise, it converts the number to a string and uses a string > comparison.

PHP 7

0 == 'foobar' // true
0 == '' // true
4 == '4e' // true (4e is cast as a number and becomes 4)

PHP 8 converts numbers to strings before making comparisons

0 == 'foobar' // false
0 == '' // false
4 == '4e' // false ('4e' is considered non-numeric therefore 4 is cast as a string and becomes '4')

This is a major change therefore it was implemented in a new major PHP version. This change breaks backward compatibility in scripts that depend on the old behavior.

This is due to how PHP does the comparison operation that the == comparison operator denotes:

> If you compare a number with a string or the comparison involves numerical strings, then each string is converted to a number and the comparison performed numerically. […] The type conversion does not take place when the comparison is === or !== as this involves comparing the type as well as the value.

As the first operand is a number (0) and the second is a string ('e'), the string is also converted to a number (see also table Comparison with Various Types). The manual page on the string data type defined how the string to number conversion is done:

> When a string is evaluated in a numeric context, the resulting value and type are determined as follows. > > If the string does not contain any of the characters '.', 'e', or 'E' and the numeric value fits into integer type limits (as defined by PHP_INT_MAX), the string will be evaluated as an integer. In all other cases it will be evaluated as a float.

In this case the string is 'e' and thus it will be evaluated as a float:

> The value is given by the initial portion of the string. If the string starts with valid numeric data, this will be the value used. Otherwise, the value will be 0 (zero). Valid numeric data is an optional sign, followed by one or more digits (optionally containing a decimal point), followed by an optional exponent. The exponent is an 'e' or 'E' followed by one or more digits.

As 'e' does not start with a valid numeric data, it evaluates to float 0.

"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;
}

The == operator will try to match values even if they are of different types. For instance:

'0' == 0 will be true

If you need type comparison as well, use the === operator:

'0' === 0 will be false

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.

You should use === instead of ==, because the ordinary operator does not compare the types. Instead it will attempt to typecast the items.

Meanwhile the === takes in consideration type of items.

• === means "equals",
• == means "eeeeh .. kinda looks like"

There's a rather handy method in PHP for validating a mix of "0", "false", "off" as == false and "1", "on", "true" as == true which is often overlooked. It's particularly useful for parsing GET/POST arguments:

filter_var( $item['price'], FILTER_VALIDATE_BOOLEAN );

It's not wholy relevant to this use-case but given the similarity and fact this is the result search tends to find when asking the question of validating (string)"0" as false I thought it would help others.

http://www.php.net/manual/en/filter.filters.validate.php

Basically, always use the === operator, to guarantee type safety.

I think it is best to show by examples I did, while running into the same weird behavior. See my test case and hopefully it will help you understand the behavior better:

// Normal comparison using the == Operator
echo (0 == "0"); // true
echo (0 == "a"); // true
echo (0 == "safta!"); // true
echo (1000 == "bla"); // false. It appears that PHP has a weird behavior only with the number / string 0 / "0" according to the past 3 examples.
echo (23 == "23"); // true. So as we said, PHP has a problem (not a problem but weird behavior) only when the number / string 0 (or "0") is present
echo (23 == "24"); // false. values aren't equal (unlike last example). The type is less relevant with the == operator as we can see.

// Now using the === and !== Operators
echo ("0" === 0); // false, since === requires both value and type to be the same. Here, type is different (int vs string)
echo ("0" !== 0); // true because they aren't the same in terms of === comparison (type is different and that's why it's true)
echo ("bla" === "blaa"); // false because the values are not the same. The type is the same, but === checks for both equal type and equal value.

//Now using casting and === Operator:
echo ((string)123 === "123"); // true. The casting of the int 123 to string changed it to "123" and now both variables have same value and are of same type
echo ((int)"123" === 123); // true. The casting of the string 123 to int, changed it to int, and now both variables are of same value and type (which is exactly what the === operator is looking for)

// Now using casting and == Operator. Basically, as we've seen above, the == care less for the
// type of var, but more to the value. So the casting is less relevant here, because even
// without casting, like we saw earlier, we can still compare string to int with the == operator
// and if their value is same, we'll get true. Either way, we will show that:
echo ((string)123 == "123"); // true. The casting of the int 123 to string changed it to "123" and now both vars have same value and are of same type
echo ((int)"123" == 123); // true. The casting of the string 123 to int, changed it to int, and now both vars are of same value and type (which is exactly what the === operator is looking for)