PHP Debugging Tips - Computer Programmers, Programming Forum

Mau · #1 01-02-2006

Set Error Reporting to E_ALL

Quote:

Mau originally posted:

This can be easily done in php.ini. To do this, simply open your php.ini file and change the value of error_reporting to E_ALL. If you do not have access your PHP configuration file, you may insert this at the top of every page:

PHP Code:


			
error_reporting(E_ALL);

Doing this will cause PHP to output extra errors, warnings, and notices--such as undefined variables. This can catch those little mistakes.

Check that MySQL Syntax

Quote:

Mau originally posted:

If you are having a problem with your MySQL statements, your SQL is most likely invalid. The best way to check your MySQL is to simply output your statement before you execute it. For example:

PHP Code:


			
$sql = 'SELECT * FROM table WHERE id='.$my_id;
echo $sql;
$result = mysql_query($sql);

By doing this, you are verifying what is actually being sent to the MySQL server. If, for example, $my_id turns out to contain a string, you would see this during output.

If, after you make some changes to your SQL it still doesn't work, you should then try using MySQL's error message. The best way to do this is to tack mysql_error(); on to the end or your statements. For example:

PHP Code:


			
$result = mysql_query($sql); or die(mysql_error());

This will output the first MySQL error the screen, which you can debug further.

Focus Your Scope

Quote:

Mau originally posted:

You should make sure that you are using the right variables in the right scope. Are you using a global variable, or a local variable? For example:

PHP Code:


			
$x = 0;
function a()
{
  $x++;
}
a();
print $x; // prints 0!

As much as you think this should print out 1, it actually prints 0. To fix it:

PHP Code:


			
$x = 0;
function a()
{
  $GLOBALS['x']++;
}
a();
print $x; // prints 1!

On a related note, are you using references? Do you intend to? You should check these.

Trace Through Your Program

Quote:

Mau originally posted:

Sometimes your code may be correct, but your logic isn't. You should do a back trace on your program to find out your mistake. Doing this is simple in PHP: You just output the contents of the key variables to the screen at critical points. You can then see for yourself what values are being passed.

Quote:

JordanF98765 originally posted:

...i find that moving a die('here'); command through the code helps me find exactly which line is responsible for strange behavior or output.

Take a Break; Think it Over

Quote:

Mau originally posted:

Sometimes you just need to take a break and walk away for an hour. Go do something different, away from the computer and programming. Usually, when you come back, you'll see your mistake and bop yourself on the head.

Check Those Silly Mistakes!

Quote:

unclekyky originally posted:

I know... Check those dang semicolons!

Quote:

Mau originally posted:

If you are getting a parse error, it means that your code is malformed. You should check thos semicolons and make sure that all quotes are placed in the proper location.

Check the Manual

Quote:

Mau originally posted:

You should check the manual at www.php.net. Type in the function that you are having trouble with and verify that you are using it correctly. This manual also contains many examples, so if you are doing something similar to the examples, check to make sure you are doing it correctly.

Ask Someone

Quote:

Mau originally posted:

If you are still having trouble, then ask! Just write your question and somebody will probably get back to you in a few days.

Further Reading

If any of you would like to share your debugging techniques, please post below. They will be added to the master list, with credit given to you.

JordanF98765 · #2 01-02-2006

A very good list of techniques. To add into the Tracing Your Code section, i find that moving a die('here'); command through the code helps me find exactly which line is responsible for strange behavior or output.

unclekyky · #3 01-03-2006

I know... Check those dang semicolons!

Mgccl · #4 01-02-2007

use dBug and debugConsole
also, you can try use
if(30 == $a);
instead of
if($a == 30);
so when you misspell "==" with "="
there will be an error...

if(30 == $a);

MaradoX- · #5 08-06-2007

PHP Code:


			
$result = mysql_query($sql); or die(mysql_error());

isnt it supposed to be?:

PHP Code:


			
$result = mysql_query($sql) or die(mysql_error());

priji · #6 01-04-2008

Watch for parser errors at the End of Feed (EOF). If the parser error line number is the very bottom of your script then you likely have a braces mismatch. These can be very difficult to spot if you’ve added a bunch of code without running the program. Here’s an example of a braces mismatch: <?php
$flag = 0;
$names = array(‘TDavid’,‘php-scripts.com’,);
foreach($names as $each) {
if($each == ‘TDavid’) {
$flag = 1;
if($flag == 1) {
print ‘I found TDavid’;
}
}
?>

Can you spot the brace mismatch above? It should be directly below the $flag=1; One way in coding that you can cut down on brace mismatches is always indent. Let’s rewrite the code above with indenting below:
<?php
$flag = 0;
$names = array(‘TDavid’,‘php-scripts.com’,);
foreach($names as $each) {
if($each == ‘TDavid’) {
$flag = 1;
}
if($flag == 1) {
print ‘I found TDavid’;
}
}
?>

Notice how the braces match up? If you indent your code you’ll keep braces closed. There are also some helper text editor programs that will watch for brace mismatches and alert you to this common error.

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