Tutorials

Debugging: A Beginner's guide

by Ben Smithers on Jun 9, 2008 5:47:39 PM

Views: 26007

Database Errors

How often have you seen this?

Warning: mysql_fetch_assoc(): supplied argument is not a valid MySQL result resource in C:\wamp\www\phpfreaks.php on line 5

The error message doesn't tell you a lot, does it? What it does tell you is that there was an error with your query. What we need is way of seeing that error. Handily, mysql_error() does just that. Now, you will see a lot of people on the forum (myself included) suggesting that to test a query you should add an or die statement and echo mysql_error() like so:

However, better practice would be to use "or trigger_error()". Why? Well, because it will save you troubles should the script go live. By throwing an error, you still get the benefit of seeing the problem whilst you are developing and have display_errors turned on, but can also handle things gracefully should the script go live and you turn of error_reporting. You'll also want to echo your query. It's so much easier to spot a syntax error when you actually look at what is being executed. This is particularly true if the query is complex and contains many variables.

Therefore, you should do this:

Notice that we create our query in a string before we execute it; this allows us to echo it.

Now, you might also have a query which does not generate an error, but also doesn't return what you expect. You may, for example, be expecting lots of rows but don't get any. Again, the best thing you can do is to echo the query:

9 times out of 10 you will find that a variable in your query doesn't contain what you expected it to and so can trace the error.

Finally, we'll talk a little bit about logical errors on the next page.

Comments

One small note worth adding: setting display_errors at runtime (ini_set()) won't work if the script contains fatal errors (like a parse error) because it will never get executed.

1. philipolson on Jun 10, 2008 2:09:56 PM

That's a good point there. I'll add a note in to that effect.

2. Ben Smithers on Jun 10, 2008 5:13:00 PM

You can also set error reporting in the .htaccess file if you have access to that and not your php.ini file. Here's what you need to put in the .htaccess file

php_flag display_errors on
php_value error_reporting 2047

// Or whatever error reporting level you want to use - I use 2047.

3. anthropos9 on Jun 10, 2008 6:55:58 PM

Re comment #1: Did you check that what you said is true? Setting it at runtine does work (providing you do it before any errors occur).

4. Daniel Egeberg on Jun 11, 2008 12:54:49 AM

With a syntax error Daniel? Not in my tests. With error_reporting turned off, this prodces a blank page:

5. Ben Smithers on Jun 11, 2008 1:40:02 AM

Hmm... I tested using

That does not output any errors. I just figured it would be the same type of fatal error as a syntax error would.

6. Daniel Egeberg on Jun 11, 2008 2:39:08 AM

Yeah, but then i think that makes sense. If there's a parse error, then the PHP engine wont be able make sense of the code; so it wont be able to understand that it's supposed to make a modification to the php.ini. Whereas with something like a fatal error for an undefined function, PHP can understand what it's being asked to do, just can't do it.

Also; valid point regarding .htaccess anthopos9. I'm not sure i'll add it in however, as it might just confuse the issue.

7. Ben Smithers on Jun 11, 2008 7:22:07 AM

Ben, with the htaccess thing... "php_flag display_errors on
php_value error_reporting 2047"

Right now theres nothing in the htaccess file but it writes errors to "error_log".
If I added in what you said would it fix it?

8. Stephen on Jun 15, 2008 8:31:49 PM

Well, it would turn turn display_errors On and it would set your error_reporting to 2047 which is all errors except recoverable. So if that's what you meant by 'fix it' then yes, i guess it would.

You might need to clarify, however.

9. Ben Smithers on Jun 16, 2008 3:51:06 PM

I'm new to php and I am reading PHP 5 for Dummies, I'm at chapter 3 creating you first PHP script. The script I'm using is
<html>
<head><title>Hello World Script</title></head>
<body>
<?php
echo “<p>Hello World!</p>”
?>
</body>
</html>
I saved it as helloworld.php on my localhost and even on my web server and I keep getting the same message:
Parse error: syntax error, unexpected '>' in C:\wamp\www\Test\helloworld.php on line 5
I have no idea whats wrong with it as I have just copied and pasted from the ebook!!
Please could someone help as I can't work it out?

10. Bleakers on Jun 19, 2008 8:51:48 AM

It appears you're using some sort of curly quotes instead of the regular straight ones. It's probably printed like that in your ebook. Try to type it manually and run it again.

11. Daniel Egeberg on Jun 19, 2008 1:17:22 PM

Thank you very much Daniel. That has fix the problem.

12. Bleakers on Jun 20, 2008 6:40:16 AM

As an addendum to the section on logic errors, an extremely handy line of code for figuring out what variables have what values (without having to exhaustively echo them) is the following:

This will exit the script - you can just as easily use echo. As a warning, it only works within the scope from which it is called (ie. from within functions only the local scope is returned), and from the global scope, you'll get all superglobals as well. This can flood you with data that you don't need - in this case, feel free to trim it down by specifying particular arrays in place of get_defined_vars().

13. akitchin on Aug 21, 2008 6:12:02 PM
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