lets say i got:

if (file_exists("users/$user.txt")) {  

Now if the $user would be:

$user = 'lool") || !file_exists("AA'; 

and the full code would be

if (file_exists("users/lool") || !file_exists("AA.txt")) {  

And it always passes it.

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code would be
if (file_exists("users/lool\") || !file_exists(\"AA.txt")) {