Check if post exists - php

Question

So i want to check if a post exists in the database but i have some problems with redirection.
This is my work so far:
echo '<br>';//the $row part tells the DB what post his looking on
echo 'View comments';
This is the show comment button that leads to the section where you see the comments for the post.
<?php
require_once ('checkp.php');
I have it to require the post checking script once.
<?php
include ('variables.php');
//connects to DB
$dbc=mysql_connect($host,$user,$pass);
if ($dbc) {
} else {
echo ('Failed to connect to MySql; '. mysql_error());
}
//selects db from MySQl
$sqldb=mysql_select_db('a2318052_blog');
$pid=$_GET['post_id'];
$query1="SELECT * FROM posts_b WHERE id='$pid'";
$sql=mysql_query($query);
if ($sql) {
} else {
echo "cant run query";
}
if (mysql_num_rows($sql) > 0) {
echo "that post does not exist!";
} else {
header ("location: comments.php?post_id='. $pid.'");
}
?>
And this is the script that checks for a empty result and then redirects back. I believe its something with the redirect here (header ("location: comments.php?post_id='. $pid.'");)

Answer 1

You mixed the quotes on the redirect:
"location: comments.php?post_id='. $pid.'"
should be
"location: comments.php?post_id=". $pid
The dot in php is used to concatenate strings. Bu there you are opening the string with " and closing it with '.
EDIT : Also as someone else already noticed you're using query instead of query1.
Also i suppose instead of:
if (mysql_num_rows($sql) > 0) {
echo "that post does not exist!";
you wanted something else:
if (mysql_num_rows($sql) == 0) {
echo "that post does not exist!";

Answer 2

You probably don't want single quotes around the post_id...
header ("location: comments.php?post_id=$pid");

Answer 3

first change pid
$pid = intval($_GET['post_id']); // for security
after that
if (mysql_num_rows($sql) == 0)
{
echo "that post does not exist!";
}
else
{
header("Location: comments.php?post_id=".$pid);
}

Answer 4

$query1="SELECT * FROM posts_b WHERE id='$pid'";
$sql=mysql_query($query);
You're using $query instead of $query1. That's probably the problem (along with the concatenation stuff other users have pointed out).
There's also a few other things, like I think you mixed up your if/else statement here:
if (mysql_num_rows($sql) > 0) {
echo "that post does not exist!";
} else {
header ("location: comments.php?post_id='. $pid.'");
}
Maybe you want the order to be reversed?
Also, you should look into avoiding SQL injection!
Sending a query with a $GET variable is pretty dangerous, as users can manipulate the URL and send malicious queries.
$pid=$_GET['post_id'];
Prepared statements are ideal, but for now, you could use mysql_real_escape_string around your $GET variable. It stops people from sending queries you really don't want done.