Unable to execute MYSQL query inside a PHP function - php

Question

PHP:
function generate_uid() {
$uid = mt_rand();
$sql = "SELECT user_id
FROM user_registration";
if (!mysql_query($sql,$con)) {
die('Error: ' . mysql_error());
}
$result = mysql_query($sql);
$availability = TRUE;
while($row = mysql_fetch_array($result)) {
if($row['user_id'] == $uid) {
$availability = FALSE;
}
}
if($availability == FALSE) {
generate_uid();
}
}
The error
Warning: mysql_query(): supplied argument is not a valid MySQL-Link resource in E:\Web Design EC\register2.php on line 8
Error:
But when i execute this function as normal php there is no error and the $uid is generated. What maybe the problem??

Answer 1

$con is not defined in the current variable scope. You can pass the connection to function as a parameter:
function generate_uid($con) {
...
mysql_query($sql, $con);
}
$uid = generate_uid($con);
Or you can use global:
function generate_uid() {
global $con;
...
mysql_query($sql, $con);
}
$uid = generate_uid();
Or, simply leave the connection variable out, and the last opened connection will be used:
function generate_uid() {
...
mysql_query($sql);
}
$uid = generate_uid();
All of those should work.
To learn more about variable scope, check out the PHP manual on the subject at:
http://www.php.net/manual/en/language.variables.scope.php

Answer 2

You are using $con in your function, but $con is not defined in your function scope.
http://php.net/manual/en/language.variables.scope.php
You need to either
a) Pass $con to your function or
b) Set $con as a global
c) Not use $con at all eg.
if (!mysql_query($sql))
however make sure that your app is only connecting to ONE database, otherwise you will run into issues because when you don't specify a connection it will use the last connection opened.

Answer 3

Inside your function, your MySQL connection doesn't exist. You either need to pass it in, or use the global keyword:
$con = mysql_connect('localhost', 'mysql_user', 'mysql_password');
function generate_uid()
{
global $con;
$uid = mt_rand();
$sql="SELECT user_id FROM user_registration";
if (!mysql_query($sql,$con))
{
die('Error: ' . mysql_error());
}
$result = mysql_query($sql);
$availability=TRUE;
while($row = mysql_fetch_array($result))
{
if($row['user_id']==$uid)
{
$availability=FALSE; }
} if($availability==FALSE) { generate_uid(); }
}

Answer 4

while($row = mysql_fetch_array($result))
You can't use that if you plan to use $row as an associative array. Use:
while($row = mysql_fetch_assoc($result))
instead.
EDIT
MYSQL_BOTH is turned on by default, forgot about that. The problem is pointed out by the other users, regarding the connection not being available in the function scope.