PHPs strpos does not work as intended with double quoted string - php

Question

I'm using the following code to return true or false if a string contains a substring in PHP 8.0.
<?php
$username = "mothertrucker"; // This username should NOT be allowed
$banlistFile = file_get_contents("banlist.txt"); //Contains the word "trucker" in it
$banlist = explode("\n", $banlistFile); // Splits $banlistFile into an array, split by line
if (contains($username, $banlist)) {
echo "Username is not allowed!";
} else {
echo "Username is allowed";
}
function contains($str, array $arr)
{
foreach($arr as $a) { // For each word in the banlist
if (stripos($str, $a) !== false) { // If I change $a to 'trucker', it works. "trucker" does not
return true;
}
}
return false;
}
?>
This is to detect if an inappropriate word is used when creating a username. So for example, if someone enters the username "mothertrucker", and "trucker" is included in the ban list, I want it to deny it.
Right now with this code, If I just type in the word "trucker" as a username, it is found and blocks it. Cool. However if there's more to the string than just "trucker", it doesn't detect it. So the username "mothertrucker" is allowed.
I discovered that if I explicitly type in 'trucker' instead of $a in the stripos function, it works perfectly. However, if I explicitly type in "trucker" (with double quotes), it stop working, and only blocks if that's the only thing the user entered.
So what I'm seeing, is it looks like the string $a that I'm passing it is being interpreted by PHP as a double quoted string, when in order for this to detect it properly, it needs to be a single quoted string. But as far as I can tell, I have no control over how php passes passing the variable.
Can I somehow convert it to a single quoted string? Perhaps the explode command I'm using in line 2 is causing it? Is there another way I can pull the data from a txt document and have it be interpreted as a single quote string? Hopefully I'm made sense with my explanation, but you can copy and paste the code and see it for yourself
Thanks for any help!

Answer 1

One potential problem would be any whitespace (which includes things like \r) could stop the word matching, so just trimming the word to compare with can tidy that up...
stripos($str, $a)
to
stripos($str, trim($a))

Answer 2

I do not know what your file actually contains so i dont know what the result of explode is.
Anyways my suggestion is (depending on the speed you want to perform this and also the length of the banlist file also your level of banning) to not explode the file and just look into it as a whole.
<?php
$username = "allow"; // This username should be allowed
$banlist = "trucker\nmotherfucker\n donot\ngoodword";
var_dump(contains($username, $banlist));
function contains($str, $arr)
{
if (stripos($arr, $str) !== false) return true;
else return false;
}
?>
Otherwise if you are going to allow say good which is an allowed word but since it is in the file with goodword it will not (using my example), you should not use stripos but instead use your example and use strcasecmp