PHP Undefined variable in functions and included s

问题:

I've read a LOT of things about this problem, however I still cannot fix it.

In my functions file I declare a variable with a value like so:

$px_host = "localhost";

And I have a database query function like so:

function dbQuery($database, $reqquery){
if(!$connect = mysql_connect($px_host, $px_dbuser, $px_dbpass)){
    exit("Error - cannot connect to MySQL server - " . mysql_error());
}

if(!$database = mysql_select_db($database)){
    exit("Error - cannot select database - " . mysql_error());
}

if(!$query = mysql_query($reqquery)){
    exit("Error - query error.");
}

return $query;
}

And whenever I try and run the function, I get an 'Undefined Variable' error. I've tried setting the variable to global, however it still says it's undefined. They are in the same file and the variable is defined and set before the function.

(I'm bad at explaining this so try and work round it) The file which contains dbQuery() is included in another file. In the other file, there is a function which uses dbQuery() to get certain information. Is it possible that because it's being initially run from the first file, the variable is outside the scope?

回答1:

In your specific case you should declare global within function all variables outside the function. So

function dbQuery($database, $reqquery){
    global $px_host,$px_dbuser,$px_dbpass;
    // rest of function
}

But your code can be improved: You should define constants at the beginning of your script

define('DB_HOST','localhost');
define('DB_USER','user');
define('DB_PASS','pass');

and use constants inside the function (so no need to declare global, and it's more logic because host, user and pass aren't variable but constants).

You should connect at database only once at the beginning of your flow so the function dbQuery execute only the query (according with the function name).

EDIT for completeness of answer:

As some users say you in other comments, I invite you to read the php doc for mysql_connect and see the red advise:

Use of this extension is discouraged. Instead, the MySQLi or PDO_MySQL extension should be used. See also MySQL: choosing an API guide and related FAQ for more information. Alternatives to this function include:

I'm not here for say you what you MUST do in your project, but read the doc and follow the tipps/suggestions is essential for the success of your project. :)

回答2:

It's getting late, so this is only a partial answer.

Another approach you could take is to pass the database instance into your helper function, thus resolving the credentials issue.

function dbQuery($database, $reqquery)
{
    if (false !== ($query = mysql_query($reqquery, $database))) {
        exit("Error - query error.");
    }

    return $query;
}

Now this function receives its dependency via the arguments and is also a lot shorter and doesn't connect / query / disconnect every time.

The remaining code has to be written elsewhere; if you require a database at every page, you can write this pretty high up the chain:

if (false === ($connect = mysql_connect($px_host, $px_dbuser, $px_dbpass))) {
    exit("Error - cannot connect to MySQL server - " . mysql_error());
}

if (false === mysql_select_db($database)) {
    exit("Error - cannot select database - " . mysql_error());
}

Then you pass $connect around wherever it's required.

$res = dbQuery($connect, 'SELECT "hello world"');

Of course, mysql_connect and friends use implied connections, so you technically don't need to pass it around anyway; it's a good pattern nonetheless.

Last but not least

Learn how to use PDO and revel in the magical world of OOP ;-)

回答3:

If you set the variable global, you will need to set it global in the function as well. In this case:

$px_host = "localhost";

function dbQuery($database, $reqquery){
    global $px_host;
    if(!$connect = mysql_connect($px_host, $px_dbuser, $px_dbpass)){
        exit("Error - cannot connect to MySQL server - " . mysql_error());
    }

    if(!$database = mysql_select_db($database)){
        exit("Error - cannot select database - " . mysql_error());
    }

    if(!$query = mysql_query($reqquery)){
        exit("Error - query error.");
    }

    return $query;
}

This should fix this problem.