String comparison using '==' or '===' vs. 'strcmp()'_问答_开发者

It seems that PHP's === operator is case sensitive. So is there a reason to use strcmp()?

Is it safe to do something 开发者_C百科like the following?

if ($password === $password2) { ... }

The reason to use it is because strcmp

returns < 0 if str1 is less than str2; > 0 if str1 is greater than str2, and 0 if they are equal.

=== only returns true or false, it doesn't tell you which is the "greater" string.

You should never use == for string comparison. === is OK.

$something = 0;
echo ('password123' == $something) ? 'true' : 'false';

Just run the above code and you'll see why.

$something = 0;
echo ('password123' === $something) ? 'true' : 'false';

Now, that's a little better.

Don't use == in PHP. It will not do what you expect. Even if you are comparing strings to strings, PHP will implicitly cast them to floats and do a numerical comparison if they appear numerical.

For example '1e3' == '1000' returns true. You should use === instead.

Well...according to this PHP bug report, you can even get 0wned.

<?php
    $pass = isset($_GET['pass']) ? $_GET['pass'] : '';
    // Query /?pass[]= will authorize user
    //strcmp and strcasecmp both are prone to this hack
    if ( strcasecmp( $pass, '123456' ) == 0 ){
      echo 'You successfully logged in.';
    }
 ?>

It gives you a warning, but still bypass the comparison. You should be doing === as @postfuturist suggested.

Always remember, when comparing strings, you should use the === operator (strict comparison) and not == operator (loose comparison).

Summing up all answers:

== is a bad idea for string comparisons.
It will give you "surprising" results in many cases. Don't trust it.
=== is fine, and will give you the best performance.
strcmp() should be used if you need to determine which string is "greater", typically for sorting operations.

Using == might be dangerous.

Note, that it would cast the variable to another data type if the two differs.

Examples:

echo (1 == '1') ? 'true' : 'false';
echo (1 == true) ? 'true' : 'false';

As you can see, these two are from different types, but the result is true, which might not be what your code will expect.

Using ===, however, is recommended as test shows that it's a bit faster than strcmp() and its case-insensitive alternative strcasecmp().

Quick googling yells this speed comparison: http://snipplr.com/view/758/

strcmp() and === are both case sensitive, but === is much faster.

Sample code: Speed Test: strcmp vs ===

strcmp will return different values based on the environment it is running in (Linux/Windows)!

The reason is the that it has a bug as the bug report says - Bug #53999strcmp() doesn't always return -1, 0, or 1

You can use strcmp() if you wish to order/compare strings lexicographically. If you just wish to check for equality then == is just fine.

Also, the function can help in sorting. To be more clear about sorting. strcmp() returns less than 0 if string1 sorts before string2, greater than 0 if string2 sorts before string1 or 0 if they are the same. For example

$first_string = "aabo";
$second_string = "aaao";
echo $n = strcmp($first_string, $second_string);

The function will return greater than zero, as aaao is sorting before aabo.

if ($password === $password2) { ... } is not a safe thing to do when comparing passwords or password hashes where one of the inputs is user controlled.
In that case it creates a timing oracle allowing an attacker to derive the actual password hash from execution time differences.
Use if (hash_equals($password, $password2)) { ... } instead, because hash_equals performs "timing attack safe string comparison".

In PHP, instead of using alphabetical sorting, use the ASCII value of the character to make the comparison.

Lowercase letters have a higher ASCII value than capitals. It's better to use the identity operator === to make this sort of comparison. strcmp() is a function to perform binary safe string comparisons. It takes two strings as arguments and returns < 0 if str1 is less than str2; > 0 if str1 is greater than str2, and 0 if they are equal. There is also a case-insensitive version named strcasecmp() that first converts strings to lowercase and then compares them.