Update: This entry seems to be getting a lot of new attention; welcome! The lessons of line length, volume, and density, along with lots of other good design principles, are applied to the Solar Framework for PHP 5. Be sure to give it a look if you're interested in well-designed PHP code.
When it comes to coding style, there are are various ideas about how you should write the individual lines of code. The usual argument is about "how long should a line of code be"? There's more to it than that, though. Developers should also take into account line volume ("number of lines") and line density ("instructions per line").
The PEAR style guide says lines should be no longer than 75-85 characters. Some developers think this is because we need to support terminals where lines may not wrap properly, or because some developer screens may not be big enough to show more than that without having to scroll sideways, or because it's tradition, and so on. These reasons may even be accurate in some sense. However, I see the 75-character rule as recognizing a cognitive limitation, not a requirement that can change with available technology.
How many words per line can a person scan, and still be able to grasp the content of the line in the context of the surrounding lines? Printing and publishing typographers figured out a long time ago that most people can read no more than 10 to 12 words per line before they have trouble differentiating lines from each other. (A "word" is counted as five characters on average.) Even allowing for a 25% to 50% increase, that brings us up to 15 words. Times 5 characters per word, that means 75 characters on a line.
So the style guide limitation on line length is not exactly arbitrary. It is about the developer's ability to effectively scan and comprehend strings of text, not about the technical considerations of terminals and text-editors.
Line Volume and Density
Some developers believe you should put as much code as possible on a single line, to reduce line-count. They say this makes the code read more like a "sentence". In doing so, these developers trade line "volume" for line "density" (or line "complexity").
Increasing the density of a line tends to make it less readable. Lines of code are generally lists of statements, not natural-language prose. If you put a lot of instructions on a single line of code, that tends to make it harder for other developers to decipher the logical flow.
Examine the following:
list($foo, $bar, $baz) = array(Zim::getVal('foo'), Dib::getVal('bar'), Gir::getVal('baz', Gir::DOOM));
(Yes, I have actually seen code like this. Only the identifier names have been changed.)
Now compare that to the following equivalent code:
$foo = Zim::getVal('foo'); $bar = Dib::getVal('bar'); $baz = Gir::getVal('baz', Gir::DOOM);
When I showed this rewrite to the initial developer, his complaint was: "But it's more lines!".
Increasing line volume ("more lines") and reducing line density does three things:
It reduces line length to make the code more readable.
Making it more readable makes the intent of the code more clear. The logical flow is easier to comprehend.
In this particular case, it may be faster than the original one-liner, because it drops the list() and array() calls. True devotees of the Zend Engine will be able to say for certain if this translates into faster bytecode execution. (I am not a fan of speed for its own sake, but in this case it would be good gravy over the meat of the above two points.)
In reducing line density, you don't have to make one line correlate with a single statement (although usually that's a good idea). Here's another way to rewrite the original example, this time as a single statement across multiple lines:
list($foo, $bar, $baz) = array( Zim::getVal('foo'), Dib::getVal('bar'), Gir::getVal('baz', Gir::DOOM) );
I find this less readable than the initial rewrite, but the principle is the same: more lines, but shorter, to improve readability.
If shorter lines are better, does that mean lines should be as short as technically possible?
$foo = Zim::getVal( 'foo' ); $bar = Dib::getVal( 'bar' ); $baz = Gir::getVal( 'baz' , Gir::DOOM );
It looks like the answer is "no". The line-volume vs. line-density argument is about readability and comprehension. The above example, while absurd, helps to show that overly-short lines are as difficult to read as over-long ones.
Developers with good style balance all the considerations of line length, volume, and density. That means they write lines of code no more than about 75 characters long, but not so short as to be increase line volume without need. They also show attention to line density for reasons related to cognition and comprehension, not merely technical syntax.