PHP General Guide
If you're reading this you're either looking for tips, or curious about what interesting stuff I can possibly add to an already amazing forum. I'm going to dispense some tips about general PHP development practices based on my experience, my experience with other developers and my experience with Coding Forums.
I will be adding to this post reguarly depending on what I've had to moderate in the last week, and comments I've recieved.
1.2. Controverial though it may seem, the community as a whole are following some vague variation of http://framework.zend.com/manual/en/...-standard.html - I don't like all of it, and I don't expect you to either. I break some of their guidelines on a daily basis, but I have 10 years experience of reading my own PHP. Try and meet as many of those guidelines as you feel able to. It's not about some facist coding law, it's about standard ways of code layout that the majority of people can read easily.
1.3. To follow on from 1.1 and 1.2: Code that's easy to read is easy to support. If you want support, make it easy for people to help you.
1.5. MVC: Read up on it, if you can. I'm not saying MVC is the best idea in the world, but it's good and it involves separating your HTML, PHP and Database specific code. This is always a great thing. Please read and consider this as it will, I promise, make your life easier later on.
1.6. Magic Quotes are the worst thing in the programming world, ever. This is a PHP feature that automatically puts a \ infront of ' and " in an effort to make SQL safer. It was a terrible idea when it was implemented. It's a terrible idea now. It will continue to be a terrible idea forever. If you actually use this feature and have no current plans to phase it out, please stand in line as there are hundreds of thousands of people wanting to slap you for it. See 2.1, 2.2 and 2.3.
1.8. XSS and htmlentities: One of the most embaressing things that can happen to your site is an XSS attack. It's cross site scripting, which means someone has put a <script src="somethingbad.js"> on your site. Once such a script is on your site, it can redirect users to malicious sites, use social engineering to get malware on your users PCs and severely reduce people's trust in your website. When displaying something that could have potentially come from a user, please make sure you run it through htmlentities() first. Please.
2. MySQL with PHP
2.1. Sanitise your SQL. Use mysql_real_escape_string() or equivalent.
2.2. Sanitise your SQL. Use mysql_real_escape_string() or equivalent.
2.3. Sanitise your SQL. Use mysql_real_escape_string() or equivalent. Yes, this is worth 3 separate points. People are far too lax with putting $_POST, $_GET, $_SESSION etc data directly into an SQL query. DO NOT DO THIS. So many sites break with an apostrophe, it's actually quite depressing. Unless you can guarantee the data you're putting into the SQL doesn't include apostophes, quotes or somesuch, you should always sanitise it.
2.4. Look for a database abstraction layer. Google will help you with this, and most of them will help you sanitise your SQL properly. Using an abstraction layer also helps if you need to move from MySQL to Postgresql or MSSQL later.
3.2. A common issue with AJAX is caching. Unless you know better for your specific case, always disable caching in your AJAX library and PHP.
4. Bash/Shell with PHP
4.1. If you're developing a portable script, you should never, ever use shell_exec(), exec(), `` etc or any method that causes a program to be run on the server. Almost every time you think you need a shell script, look again and see if it's really necessary. You'd be surprised what can be accomplished in pure PHP, or with the help of a PECL addon. (Thanks to oesxyl for adding to this).
4.2. When running a command though one of the many PHP functions provided, I don't insist, I demand you run each and every parameter through escapeshellarg(). You'd be surprised how many websites are vulnerable to having their whole website wiped through some careless code.
4.3. File permissions are aren't just an annoyance, they're a useful tool to help against attacks like in 4.2. For most people, you want 0444 for a file you want readable but not writable or executable, 0666 is readable and writable by everyone. Directories have to be executable - executable for a directory means you can view the contents, so 0555 (readable and executable) or 0777 (read, write and execute). The first digit says "this is octal", nevermind if you don't understand octal. The second digit is for the file owner, the third for the file group, the fourth for everyone else. It's often dependant on the use of extentions such as SuExec and SuPHP. This is a complex topic that I may cover later in a more appropriate forum.
5.1. Installing and using a debugger might take you an hour or two to get right. It'll save you twice that in the first week. Xdebug is a great tool, and having met the author, I can tell you he's a nice guy too! There are many editors that work with this. I have used Komodo IDE (my personal choice, though it's commercial), Netbeans, Aptana and Eclipse PDT and they all work reasonably well. I believe Komodo IDE is the best debugging IDE if you can afford it.
5.2. When debugging, remember to turn on "display_errors" and "error_reporting()", or check your logs. If you have shell access, "tail -f /path/to/your.log" is a handy way to watch your logs. (Thanks to oesxyl for this tip).
5.3. Remember, MySQL can be a black-hole of errors. Always check mysql_error() or mysql_errno() after your queries. Point 2.4 can help with this by converting MySQL errors into PHP errors or exceptions. (Thanks to oesxyl for this).