PHP - How To Empty Variable If It Doesn't Begin With Http?

I have this line in my code

$result2 = mysqli_query($link, 'Select * from categories where cat_loc ='.$nav['id']);

how do I say "if $result2 is empty or false"? (basically if it didn't find any cat_loc = to $nav['id'])

 to echo '</li>';

I'll try to explain this as easily as possible. 
I'm working on making a little link shortener site just for fun/practice as a beginner with php.
I originally used the source code 'lilurl' as a base to start out, but now I'm adding stuff to it for actual practice.
I need some help though.
I have two files, my index and my external php file.  The external file is called on at the beginning of the index with require_once.
The user submits a url into the PHP_SELF form and a url is returned, and it works all great.  Now I'm trying to add the option to make a custom url suffix. 
Code: [Select]
<form action="<?php echo $_SERVER['PHP_SELF']?>" method="post">

<fieldset>
<label for="longurl">Enter a URL:</label>
<input type="text" name="longurl" id="longurl" />
<input type="text" name="suffix" id="suffix" />
<input type="submit" name="submit" id="submit" value="Shrink!" />
</fieldset>

</form>[/code]
The form looks like this.  I added the 'suffix' textbox. 

So after the user submits the code, it goes back up to the top (phpself).
A variable is set for the url the user has inputted as "$longurl", like this.
Code: [Select]
$longurl = trim(mysql_escape_string($_POST['longurl']));So I added this.
Code: [Select]
    $suffix = ($_POST['suffix']);And to check if it worked, I went down the the bottom of the index and had it echo the suffix variable.  When I entered in code into the suffix text box, it would be echoed at the bottom of the page, so the variable did receive the value. 

So here's where the external file comes in. 
It is called on by require_once at the very beginning of the index.  It is full of functions.  One of the functions generates a random string of numbers (that I coded) to be used as the url suffix (uses the $id variable).  So my plan was to erase the $id = $rand1 . blah blah stuff and put $id = $suffix.  $suffix being the variable from the index that the user inputted.  Make sense?  Well whenever I try it, it always acts the like the variable is empty, even when I echo it on the index it returns the value of the textbox.  Does anyone know what I'm doing wrong?  Could it be because since I call on $suffix in the external file, and it's only been declared in the index that it's treating it like a new/empty variable? 

[spoiler]
Someone also explained it to me like this.
-I call upon the external file.
-I declare the variable is the value of the text box.  (Which means it's currently empty because the form has not been submitted.)
-Then, the form is submitted as PHP_SELF.

-The external file is called upon again.
-The variable is then assigned a value. 
It's hard to explain, but basically that the variable in the external file is not told about the new value of the variable because they are declared after the external file is called upon? :\

So I tried moving the  $suffix = ($_POST['suffix']); before the external file is called upon, but the same thing happens.  It's hard to explain the situation, but I've tried a lot of things and can't get it to work.[/spoiler]

i am pulling the login date from the databases and i want it to check if the variable is empty and if it is echo Never Loged In and if it not empty echo the date
          <?php
          include'db.php';
  $id=$_GET['id'];
  $sql=mysql_query("SELECT * FROM admin WHERE id='$id'")or die(mysql_error());
  while($row=mysql_fetch_array($sql))
  {
  $date=$row['login_date'];
  if($date > 0)
  {
  echo"Never Loged In";
  }
  else
  {
  echo"$date";
  }
  }
  ?>

Hello all,

I am a beginner at PHP and trying to form an IF statement, I'm running into a problem which I cannot seem to solve.
Due to my lack of syntax knowledge I would appreciate if someone could point me in the right direction.

The combo variables are sent to this page through a form and defined as these variables at the top of the page.
e.g $combo0 = $_POST["combo0"];

When the results are displayed any form field which was left blank or empty I want to show "N/A" if data is detected in being sent through the form I then want the "Else" statement to take affect.

Currently when I run the below statement N/A is echoed despite if data was sent through the form fields or not....

if (empty($scrap))  {
echo "<h3>Submitted Scrap Results</h3> No scrap parts were booked off";
}
elseif (empty($combo0) || empty($combo1) || empty($combo2) || empty($combo3) || empty($combo4)) {
echo "N/A";
}
else {
echo "<h3>Submitted Scrap Results</h3> <b>Reason:</b> " .$combo0." - ".$combo1." - ".$combo2." - ".$combo3." - ".$combo4."
<br /> <b>on machine</b> " .$mid. " - <b>No. Scrap</b> " .$noscrap. "";
}

Anyone any ideas?

I know the following simple demonstration fails and what to do to rectify it (though with a lot of additional code is necessary), but I don't know why this is the behaviour. It would be great if someone could explain to me why, and what the neatest way of fixing it would be.

Code: [Select]
+----------+--------------+------+-----+---------+
| Field    | Type         | Null | Key | Default |
+----------+--------------+------+-----+---------+
| Id       | int(11)      | NO   | PRI | NULL    |
| Name     | varchar(45)  | YES  |     | NULL    |     
| Number   | int(9)       | YES  |     | NULL    |         

Code: [Select]
$name = "Bradley Cooper";
$number = "";

$query = "INSERT INTO table (Id, Name, Number) VALUES (1, '$name', $number)";
mysql_query($query);

Because the $number variable is empty, this simple query fails. Logically, one would think that nothing or NULL gets inserted to that field since the MySQL structure rule states that default is NULL, and since nothing is fed to it in the query, I think that it would automatically be NULL since if the column is omitted, that is exactly the value that will be inserted.

Enclosing the variable with quotes is not preferable since this field accepts integers. Also, converting the variable to an integer by (int)$number results in a 0, which is not what I want either. Checking if the variable is empty of not, and if it is, assign $number = "''" so that the query would succeed by enclosing the empty string with single quotes. But this creates a lot of unnecessary code (some of my tables are 80 columns wide and the query therefore have equal amount of variables).

So to summarise, why is this happening and how do I neatly avoid assigning quotes to the empty strings? In other words, how do I insert nothing when variables that usually contain an integer is on occation empty?