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Wie das im Netz so ist: der ursprüngliche Link zum Artikel existiert nicht mehr (reinholdweber.com hat bei seiner Blog-Aktualisierung sämtliche alte Posts weggeworfen), aber es gibt reichlich Repliken davon - vielleicht ist der eigentliche Autor auch ein ganz anderer. Jetzt ist es ein Repost mehr. Die Aussagen sind von unserer Seite nicht getestet. Speziell mit Faktoraussagen muss man vorsichtig sein, auch weil die Liste nicht in allen Punkten aktuell ist:

1. If a method can be static, declare it static. Speed improvement is by a factor of 4.
2. echo is faster than print.
3. Set the maxvalue for your for-loops before and not in the loop.
4. Unset your variables to free memory, especially large arrays.
5. Avoid magic like __get, __set, __autoload
6. require_once() is expensive
7. Use full paths in includes and requires, less time spent on resolving the OS paths.
8. If you need to find out the time when the script started executing, $_SERVER['REQUEST_TIME'] is preferred to time()
9. See if you can use strncasecmp, strpbrk and stripos instead of regex
10. str_replace is faster than preg_replace, but strtr is faster than str_replace by a factor of 4
11. If the function, such as string replacement function, accepts both arrays and single characters as arguments, and if your argument list is not too long, consider writing a few redundant replacement statements, passing one character at a time, instead of one line of code that accepts arrays as search and replace arguments.
12. Error suppression with @ is very slow.
13. $row['id'] is 7 times faster than $row[id]
14. Error messages are expensive
15. Do not use functions inside of for loop, such as for ($x=0; $x < count($array); $x) The count() function gets called each time.
16. Incrementing a local variable in a method is the fastest. Nearly the same as calling a local variable in a function.
17. Incrementing a global variable is 2 times slow than a local var.
18. Incrementing a object property (eg. $this->prop++) is 3 times slower than a local variable.
19. Incrementing an undefined local variable is 9-10 times slower than a pre-initialized one.
20. Just declaring a global variable without using it in a function also slows things down (by about the same amount as incrementing a local var). PHP probably does a check to see if the global exists.
21. Method invocation appears to be independent of the number of methods defined in the class because I added 10 more methods to the test class (before and after the test method) with no change in performance.
22. Methods in derived classes run faster than ones defined in the base class.
23. A function call with one parameter and an empty function body takes about the same time as doing 7-8 $localvar++ operations. A similar method call is of course about 15 $localvar++ operations.
24. Surrounding your string by ' instead of " will make things interpret a little faster since php looks for variables inside "..." but not inside '...'. Of course you can only do this when you don't need to have variables in the string.
25. When echoing strings it's faster to separate them by comma instead of dot. Note: This only works with echo, which is a function that can take several strings as arguments.
26. A PHP script will be served at least 2-10 times slower than a static HTML page by Apache. Try to use more static HTML pages and fewer scripts.
27. Your PHP scripts are recompiled every time unless the scripts are cached. Install a PHP caching product to typically increase performance by 25-100% by removing compile times.
28. Use memcached - memcached is a high-performance memory object caching system intended to speed up dynamic web applications by alleviating database load
29. Use ip2long() and long2ip() to store IP addresses as integers instead of strings in a database. This will reduce the storage space by almost a factor of four (15 bytes for char(15) vs. 4 bytes for the integer), make it easier to calculate whether a certain address falls within a range, and speed-up searches and sorts (sometimes by quite a bit).
30. Partially validate email addresses by checking that the domain name exists with checkdnsrr(). This built-in function checks to ensure that a specified domain name resolves to an IP address. A simple user-defined function that builds on checkdnsrr() to partially valid email addresses can be found in the user comments section in the PHP docs. This is handy for catching those occasional folks who think their email address is 'joeuser@wwwphp.net' instead of 'joeuser@php.net'.
31. If you're using PHP 5 with MySQL 4.1 or above, consider ditching the mysql_* functions for the improved mysqli_* functions. One nice feature is that you can use prepared statements, which may speed up queries if you maintain a database-intensive website. Some benchmarks.
32. Learn to love the ternary operator.
33. If you get the feeling that you might be reinventing the wheel during a project, check PEAR before you write another line. PEAR is a great resource that many PHP developers are aware of, yet many more are not. It's an online repository containing over 400 reusable snippets that can be dropped right into your PHP application. Unless your project is trully unique, you ought to be able to find a PEAR package that saves at least a little time. (Also see PECL)
34. Automatically print a nicely formatted copy of a page's source code with highlight_file().This function is handy for when you need to ask for some assistance with a script in a messageboard, IRC, etc. Obviously, some care must be taken not to accidently show your source when it contains DB connection information, passwords, etc.
35. Prevent potentially sensitive error messages from being shown to users with the error_reporting(0) function. Ideally error reporting should be completely disabled on a production server from within php.ini. However if you're on a shared webhost and you aren't given your own php.ini, then your best bet is to add error_reporting(0); as the first line in each of your scripts (or use it with require_once().) This will prevent potentially sensitive SQL queries and path names from being displayed if things go awry.
36. Use gzcompress() and gzuncompress() to transparently compress/decompress large strings before storing them in a database. These built-in functions use the gzip algorithm and can compress plaintext up to 90%. I use these functions almost everytime I read/write to a BLOB field within PHP. The only exception is when I need full text indexing capabilities.
37. Return multiple values from a function with 'by reference' parameters. Like the ternary operator, most PHP developers who come from a more formalized programming background already know this one. However, those whose background is more HTML than Pascal, probably have wondered at one time "how do I get multiple values back from a function I wrote, even though I can only use one return value?" The answer is that you precede a variable with "&" and use it 'by reference' instead of 'by value'.
38. Fully understand "magic quotes" and the dangers of SQL injection. I'm hoping that most developers reading this are already familiar with SQL injection. However, I list it here because it's absolutely critical to understand. If you've never heard the term before, spend the entire rest of the day googling and reading.
39. When working with strings and you need to check that the string is either of a certain length you'd understandably would want to use the strlen() function. This function is pretty quick since it's operation does not perform any calculation but merely return the already known length of a string available in the zval structure (internal C struct used to store variables in PHP). However because strlen() is a function it is still somewhat slow because the function call requires several operations such as lowercase & hashtable lookup followed by the execution of said function. In some instance you can improve the speed of your code by using a isset() trick.

Ex.
if (strlen($foo) < 5) { echo "Foo is too short"; }
vs.
if (!isset($foo{5})) { echo "Foo is too short"; }

Calling isset() happens to be faster then strlen() because unlike strlen(), isset() is a language construct and not a function meaning that it's execution does not require function lookups and lowercase. This means you have virtually no overhead on top of the actual code that determines the string's length.
40. When incrementing or decrementing the value of the variable $i++ happens to be a tad slower then ++$i. This is something PHP specific and does not apply to other languages, so don't go modifying your C or Java code thinking it'll suddenly become faster, it won't. ++$i happens to be faster in PHP because instead of 4 opcodes used for $i++ you only need 3. Post incrementation actually causes in the creation of a temporary var that is then incremented. While pre-incrementation increases the original value directly. This is one of the optimization that opcode optimized like Zend's PHP optimizer. It is a still a good idea to keep in mind since not all opcode optimizers perform this optimization and there are plenty of ISPs and servers running without an opcode optimizer.

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