Create a new PHP page called isset.php.  Put this in the PHP block just under the h1 element in your HTML:

if($value<10){
    echo "Not enough";
} else {
    echo "OK";
}

Save, upload and try.  Hopefully you expected the error (if not you should look at the code and understand it).  In this case it is a coding error but imagine a page where the user was suppose to do something which causes $value to exist (maybe start something which is timed).  You might need to check the value every now and then to see if it has reached a limit but if you check it before it has started the user sees the warning which you do not want.  You will see a better example of this very soon.  For now this is the correct way to check if a variable exists in PHP:

if(isset($value)){
    if($value<10){
        echo "Not enough";
    } else {
        echo "OK";
    }
}

Now the original if/else will only run if the variable exists (is set).  Save, upload and try.  Nothing should show.  Now add this line before the others (just above the isset line):

$value=45;

Try it and you should get a message.  Feel free to change the number to test that the second if statement is working properly.

Unset

Under the line you just added put this line so that the variable is first created and then destroyed (before the rest of the code):

unset($value);

Try it.  That un-sets the variable so that it no longer exists.