CODEWITHSUNDEEP
phpstringcastingstrstr
laurent
laurent

Reputation: 90766

Why is this call to strstr not returning false?

I've stumbled upon an issue with strstr in an old legacy codebase. There's lot of code, but basically the test case would come down to this:

$value = 2660;
$link = 'affiliateid=1449&zoneid=6011&placement_id=11736&publisher_id=1449&period_preset=yesterday&period_start=2017-03-27&period_end=2017-03-27';

var_dump(strstr($link, $value));

I would expect this to return false since "2660" is not in the string however it returns d=1449&zoneid=6011&placement_id=11736&publisher_id=1449&period_preset=yesterday&period_start=2017-03-27&period_end=2017-03-27.

I realise that $value should be a string but still I don't understand why it's not casted to a string by PHP and why it's finding this number in the link.

Actually, if I try with $value = '2660'; it returns false as expected.

Any idea what's happening?

Upvotes: 16

Views: 2463

Answers (2)

Juan Cortés
Juan Cortés

Reputation: 21082

Short answer

When you run strstr($str, 2660) the $needle is resolved to the character "d" by calling chr(2660) and therefore it stops at the first "d" found in the given $str, in this case right at the 11th character.

Why are we calling chr(2660)?

Because when the $needle is not a string strstr casts that argument to an integer and uses the corresponding character for that position from the extended ASCII code where chr(2660) is "d".

If needle is not a string, it is converted to an integer and applied as the ordinal value of a character.

But why does chr(2660) return "d" when "d" is ord(100)?

Because values outside the valid range [0-255] will be bitwise and'ed with 255, which is equivalent to the following algorithm[source]

while ($ascii < 0) {
    $ascii += 256;
}
$ascii %= 256;

So, 2660 becomes 100, and then when passed to strstr it's used as the ordinal value of the character and looks for character "d".

Confusing? Yes. I also expected it to be casted to a string, but that's what we get for assuming things in programming. This, at least, is documented; you'd be surprised the amount of times something weird happens and there's no official explanation to be found. At least not as easily as following the same link you provided.

Why is it named strstr?

I did a little bit of research and found this gem (Rationale for American National Standard for Information Systems - Programming Language - C) from all the way back 1989 where they named all the functions relating to strings with the prefix str which is logical, then since PHP's source is written in C it will explain why it has carried. My best guess is that we are searching for a string inside another string, they do say:

The strstr function is an invention of the Committee. It is included as a hook for efficient substring algorithms, or for built-in substring instructions.

Useful docs

  • Documentation for strstr
  • Documentation for chr
  • PHPwtf a good resource for weirdness

Upvotes: 38

walther
walther

Reputation: 13600

I think this answers your question:

needle
If needle is not a string, it is converted to an integer and applied as the ordinal value of a character.

http://php.net/manual/en/function.strstr.php

Edit because of the comments:

chr(2660) returns character d, which is indeed in the haystack and that's why it won't return false as you expected.

Upvotes: 8

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