substr gives empty results in PHP - php

Question

I want to get the last 41 letters of a string, but every time its giving me a empty result.
The string is following:
02674bf7c88c9a025029f96d046651cfsingle100OK4917668157533262-07119519DEE0715DD708E6B65BC2412E1A8EE571F
Here is my code:
$urlstring = $_GET["o"];
$password = "bla";
$salt = "bla";
$result = decode($password, $urlstring, $salt);
if (strpos($result,'OK') !== false) {
$uservar = substr($result, -41);
$file = 'file.txt';
file_put_contents($file,$uservar);
}

Answer 1

Actually
$uservar = substr($result, -41);
does that, i.e. it will return last 41 chars. The problem most likely is that your decode function doesn't work and you never enter the if
$result = decode($password, $urlstring, $salt);
if (strpos($result,'OK') !== false) { // check if $result contains OK
$uservar = explode('-', $result);
$file = 'file.txt';
file_put_contents($file, $uservar[1]);
}

Answer 2

Change the folder permission to 777, and test the code again.

Answer 3

There are several ways to get the last part of the $result string provided in the question.
a) substr()
The function substr() is the most straightforward way to do it (as you already did).
$uservar = substr($result, -41);
b) regular expressions: preg_replace()
You can use preg_replace() to match the last 41 characters of the string into a group and replace the entire string with the group:
$uservar = preg_replace('/^.*(.{41})$/', '$1', $result);
The regular expression used for search:
^ is the beginning of the string;
.* is any character (.), zero or more times (*);
(.{41}) is any character (.), 41 times ({41}); the parentheses around the expression make a subpattern that can be used later in the regex or in the replacement string (as $1);
$ is the end of the string.
It matches any string that is at least 41 characters length; its last 41 characters are grouped in the first subpattern.
The replacement string ($1) refers to the 1st subpattern of the regex. It makes preg_replace() replace the matched string (the entire string) with its first subpattern (its last 41 characters). Read more about the PCRE patterns syntax.
c) convert the string to array of characters
Convert the string into an array of characters (str_split()), extract the correct slice of array (array_slice()), join the characters back (implode()).
$uservar = implode('', array_slice(str_split($result), -41));
This solution works the same as substr().
d) split the string into pieces using delimiter
If you are sure that the part of the string you need to extract does not contain any dash (-) and it starts right after the last dash then we don't even need to know its length. Use explode() to split $result into pieces delimited by dash (-) and the last piece is the part you need:
// Split the string into pieces
$pieces = explode('-', $result);
// Remove the last item from $pieces and return it
$uservar = array_pop($pieces);
// Alternatively, without modifying $pieces:
$uservar = end($pieces);
// Or
$uservar = $pieces[count($piece)-1];
e) ...
With little imagination, more complicated ways to extract a part of the string can be produced in a couple of lines of code.
Conclusion
Given $result is a not-empty string whose length is at least 41, all the above should work and produce the same result. The preg_replace() solution won't work for shorter strings (can be easily changed to work), the explode() solution doesn't care about 41 but uses - to separate the pieces.
If none of them works then your problem is somewhere else. Check for file rights, check if the code enters the if branch etc.