PHP finding string and validating it

Question

I have a database table where events are described by users (column "description") and tagged (column "tag"). I want to check that if a certain string of words appears in the "description", some tags are invalid.

Here is what I did so far:

$sql = "SELECT * FROM events WHERE user='{$_SESSION['username']}'";
$qsql = mysql_query($sql) or die ("Error Query [".$sql."]");
while($result = mysql_fetch_array($qsql)){
   $description = mysql_real_escape_string(strtolower($result["description"]));
   $a = "string I want to find";
   $aa = strpos($description, $a);

     if (($aa !== false) AND $result["tag"]!=='1'){
     echo "The string you wrote doesn't allow the tag you used.";
     }
}

The code above works. As you can see, if the given tag is not 1, an error message is echoed.

However, I am having problems when I need to validate the string against TWO OR MORE tags:

if (($aa !== false) AND ($result["tag"]!=='1' OR $result["tag"]!=='2')){
echo "The string you wrote doesn't allow the tag you used.";
}

The error is echoed even if tags 1 or tag 2 are given. I can't understand what is wrong with this.

Answer 1

If you use 'OR', it means "Show the error if its not 1 Or its not 2". If ['tag'] is '1', then it is not '2', and if ['tag'] is '2' then it is not '1', so it will always be true. Changing it to 'AND' will solve this.

Your code should be:

if (($aa !== false) AND $result["tag"]!=='1' AND $result["tag"]!=='2'){
  echo "The string you wrote doesn't allow the tag you used.";
}