Multiple Criteria Search not working

Hi,
I am trying test a multiple criteria search with two text boxes. That is user will enter values in txt1 and txt2 (or) either one. I want to execute the Select query according to the txt1, txt2 values.

If both are filled my 'where' condition will be like 'where txt1=$abc and txt2=$cde'. If anyone of the text box is empty (e.g. txt1 is empty) where condition will be 'where txt2=$cde'.

I have tried this in php with mysql as backend. But getting 500 Internal Server Error.

How to solve this.. I have added the code below

<?php
session_start();
$check=$_session['user'];
$con = mysql_connect('localhost','my_user','my_pass'); 
mysql_select_db('project', $con); //Replace with your MySQL DB Name
$txt1=$_POST['txt1'];
$txt2=$_POST['txt2'];
$sql ="select * from project_master where "
if(!empty('$txt1'))
{
$sql .= "docno='$txt1' and ";
}
if(!empty('$txt2'))
{
$sql .= "itemno='$txt2' and ";
}
$sql = substr($sql,0,-4); \\ to knock off the last and
$sql .= " order by `docno`";
while($result = mysql_fetch_array($sql)){
echo $result[7];
echo $result[3];
}
?>

Code (markup):

 

500 Errors are irrelevant to SQL coding. You should consult your server administrator.

 

Its working great when i use only

$sql ="select * from project_master";