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Leo en Anieto2k un interesante artículo donde se definen 42 truquillos que nos ayudarán a mejorar el rendimiento de nuestras aplicaciones desarrolladas en PHP.

A continuación las listo (in english) :

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. Use echo's multiple parameters instead of string concatenation.
4. Set the maxvalue for your for-loops before and not in the loop.
5. Unset your variables to free memory, especially large arrays.
6. Avoid magic like __get, __set, __autoload
7. require_once() is expensive
8. Use full paths in includes and requires, less time spent on resolving the OS
   paths.
9. If you need to find out the time when the script started executing,
   $_SERVER[’REQUEST_TIME’] is preferred to time()
10. See if you can use strncasecmp, strpbrk and stripos instead of regex
11. str_replace is faster than preg_replace, but strtr is faster than
    str_replace by a factor of 4
12. 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.
13. It's better to use select statements than multi if, else if, statements.
14. Error suppression with @ is very slow.
15. Turn on apache's mod_deflate
16. Close your database connections when you're done with them
17. $row[’id’] is 7 times faster than $row[id]
18. Error messages are expensive
19. Do not use functions inside of for loop, such as for ($x=0; $x <
    count($array); $x) The count() function gets called each time.
20. Incrementing a local variable in a method is the fastest. Nearly the same as
    calling a local variable in a function.
21. Incrementing a global variable is 2 times slow than a local var.
22. Incrementing an object property (eg. $this->prop++) is 3 times slower
    than a local variable.
23. Incrementing an undefined local variable is 9-10 times slower than a
    pre-initialized one.
24. 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.
25. 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.
26. Methods in derived classes run faster than ones defined in the base class.
27. 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.
28. 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.
29. 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.
30. 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.
31. 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.
32. Cache as much as possible. Use memcached - memcached is a high-performance
    memory object caching system intended to speed up dynamic web applications by
    alleviating database load. OP code caches are useful so that your script does
    not have to be compiled on every request
33. 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 an 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.

34. 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.
35. Not everything has to be OOP, often it is too much overhead, each method and
    object call consumes a lot of memory.
36. Do not implement every data structure as a class, arrays are useful, too
37. Don't split methods too much, think, which code you will really re-use
38. You can always split the code of a method later, when needed
39. Make use of the countless predefined functions
40. If you have very time consuming functions in your code, consider writing
    them as C extensions
41. Profile your code. A profiler shows you, which parts of your code consumes
    how many time. The Xdebug debugger already contains a profiler. Profiling shows
    you the bottlenecks in overview
42. mod_gzip which is available as an Apache module compresses your data on the
    fly and can reduce the data to transfer up to 80%

Interesantes verdad!!