从链接传递PHP值

I am new to PHP and learning. I'm trying to pass a value through a url link but it doesn't seem to work.

The link value I am passing is http://www.mysite.com/index.php?id=f

I want to run a js script if ID not F seen below but right now when I run it. It doesn't do anything:

<?php
$ShowDeskTop = $_GET['id'];
if (isset($ShowDeskTop)){
    echo $ShowDeskTop;

    if ($ShowDeskTop != "f"){
       echo "ShowDeskTop Value is not F";
               echo "<script type=\"text/javascript\">";
       echo "if (screen.width<800)";
       echo "{";
       echo "window.location=\"../mobile/index.php\"";
       echo "}";
       echo "</script>";  
    };
};
?>

I know this is easy PHP 101 but I can't figure it out. I have tried everything from w3schools to other sites on Google for the answer and having no luck. Could someone please tell me what I am doing wrong?

Thank you!

This is never gonna work since you set the variable AFTER checking if it exist.. The most easy way:

<?php
if (isset($_GET['id'])) {
    echo $_GET['id'];

    if ($_GET['id'] != 'f') {
?>
<script type="text/javascript">
    if (screen.width < 800) {
        window.location = "../mobile/index.php";
    }
</script>
<?php
    }
}
?>

I don't think <> is valid in PHP (it is in VB.NET ..) the is not operator is != or !== (strict/loose comparison).

Also you don't have to close if statements with a ;

This:

if (expr) {

}

Is valid and not this:

if (expr) {

};

I thought about writing != instead of <>.

$ShowDeskTop is not the same as $ShowDesktop variables names are case sensitive!

Fist thing, you need ; at the end of echo $ShowDesktop

And, what does f mean in if ($ShowDeskTop <> "f"){

use strcmp() instead of <> operator.

Try

if(!strcmp($ShowDeskTop, "f")){
   echo "YES, the id doesn't = f";
}

You have a number of problems including bad variable case (i.e. variables not matching), checking for variables before they exist, etc. You can simply do something like this:

if (!empty($_GET['id'])) { // note I check for $_GET['id'] value here not $ShowDeskTop
    $ShowDeskTop = $_GET['id']; 
    echo $ShowDeskTop; // note I change case here

    if ($ShowDeskTop !== "f"){  // note the use of strict comparison operator here
       echo "YES, the id doesn't = f";
       echo "<script type=\"text/javascript\">";
            echo "if (screen.width<800)";
            echo "{";
            echo "window.location=\"../mobile/index.php\"";
            echo "}";
       echo "</script>";
    } // note the removal of semicolon here it is not needed and is bad coding practice in PHP - this is basically just an empty line of code
} // removed semicolon here as well

<?php

   $ShowDeskTop = $_GET['id'];     // assign before checking

   if (isset($ShowDeskTop)){

    //echo $ShowDeskTop;             

    if ($ShowDeskTop !== "f"){
       echo "YES, the id doesn't = f";
       echo "<script type='text/javascript'>";
            echo "if (screen.width<800)";
            echo "{";
            echo "window.location.replace('../mobile/index.php');"; // assuming your path is correct
            echo "}";
       echo "</script>";
    }
   }
 ?>