As described in the Code Guidelines you need to have a editor with syntax highlighter as this will help you debug code. See the Code Guidelines for links.

• Basic principles for all programming languages:
  
  
  
  • You always need to echo/messagebox variables to see whether they are correct
  
  
  • Always make sure you close all brackets and quotes, and don't forget to escape quotes within a string
  
  
  • Make sure you use the correct operators and expressions, think about what variables need to be compared or assigned to values
  
  
  • First look through your code when it isn't working and try googling it, before you start a thread on any forum.
  
  



• PHP
  
  
  
  • The error "Warning: Cannot modify header information - headers already sent by ..." is caused because you already executed some code before you used the function header() or session_start()
  
  
  • When you get an error at the last line of the script or the error Parse error: syntax error, unexpected $end", at 99% of the cases you forgot to close an bracket }
  
  
  • The error "Parse error: syntax error, unexpected '"', expecting ',' or ';'" is caused due to the fact you messed up the quotes
  
  
  
  

• ASP.NET
  
  
  • I personally do not have experience in programming with ASP.NET, if someone that does could write a comment with some tips, I would be happy to add them.

• (X)HTML
  
  
  
  
  • Make sure you that you put arguments within events such as onclick='DoSomething("argument1", 3)' with correct quotes
  
  
  • Try to indent child elements. Example:
    
    <ul>
    <li>Text</li>
    </ul>
    
    Should be:
    
    <ul>
    <li>Text</li>
    </ul>
  
  
  • Use DOCTYPE headers and all other required headers (see http://w3c.org)
  
  
  • Always make sure your XHTML passes the W3C validator.
  
  
  
  

• MySQL
  
  
  
  • The error "XXX is not a valid MySQL resource" means you screwed up the query
  
  
  • If you can't seem to figure out why a query is not working, echo the query and execute it in PHPMyAdmin (this is a musthave!)
  
  
  
  

• JavaScript
  
  
  
  • You can use the FireBug add-on to view the (X)HTML generated by JavaScript of a specfic element.
  
  
  • I can't say this enough: each JavaScript line must end with a ;
  
  
  • Add a alert at the bottom of your script (e.g. alert("Script is working!") ) to make sure that no typo's were made.
  
  
  • Put alert's everywhere in your script, to check what code is being executed. If something is not working the alerts after that code will not be executed -> you can isolate the problem
  
  
  
  

If you have any suggestions for code debugging, post a comment below and I will consider adding them to the guide!