When building some of my PHP apps, a lot of the functionality could be coded using PEAR/PECL modules, however, the fact that some people using it may not have the access to install things, It poses a puzzler for me.
Should I forsake some users to use PEAR/PECL for functionality, where these will allow me to have a system coded up quicker than if I wrote my own functionality, but eans that it will exclude certain people from using it.
My suggestion is to start with assuming PEAR/PECL modules, and get the rest of the code done. Then, once you've got most of your code working the way you want, you can evaluate going back and piece by piece replacing the outside code with your own. Plus, by then you'll have a better idea of the impact using those has on your userbase.
Code it initially using PEAR/PECL and if you get people asking for a non PEAR/PECL version, start coding your own alternatives then for such a version.
The initial development will go much faster with this, and you may find that no-one cares about requiring 3rd party libraries once you have started releasing apps.
What I do most times is I'll never use PEAR installed globally on a server. Versions can change and affect your application.. Instead I have a config file (in my case XML) that lists all the packages required and their versions. The installer connects to my personal FTP repository and downloads and installs all the PEAR packages locally in $PROJECTBASE/lib/pear/ .. And PEAR is run locally instead of globally. Something you may want to consider.
Using PEAR is no problem, if users do not have root access to their webserver, they can simply download the PHP files from pear.php.net and add it to their include path. PECL's a little more tricky to work around, since there's often no way to install new modules without root access.
Save on development time by developing with the pear libraries, and provide the libraries bundled in what you distribute (though you'll have to make sure it obeys licensing requirements)
I would not depend on certain PECL extensions being installed unless you're doing something particularly related to one (say an XDebug web-frontend or something), the majority of installs will be carrying a fairly vanilla set of extensions.
You need to watch out because a lot of modules in pear are really of pretty low quality.
Some are great, don't get me wrong, but don't assume that anything in pear, by virtue of being in pear, is at any given quality. Which means you need to at least skim the source of a pear module before deciding to use it, which for simple enough tasks may take more time than going without pear.
pecl is different, however. Extensions tend to be better vetted and tested, else they'd crash php.
It partly depends on how much time you have, and the purpose of the project. If you're just trying to make something that works, go with PEAR/PECL. If you're trying to learn to be a better programmer, and you have the time, then I'd recommend taking the effort to write your own versions. Once you understand the innards of whatever you're trying to replace, you may want to switch to the PEAR/PECL version so that you're not wasting time reimplementing what has already been implemented...
...but on the other hand, preexisting tools don't always do exactly what you need, and sometimes have overhead that doesn't do you any good. This is why Unix command-line tools are so small and narrow of purpose; nobody really needs a version of 'ls' that can do anything besides what 'ls' can currently do. Your version of whatever PEAR library will, by virtue of being written by you, do exactly what you need doing. It requires some careful thought...
...but on the gripping hand, don't spend too much time thinking about it. Spend five minutes, make a decision, and start coding. Even if you make the wrong decision, you'll at least have gotten more practice coding. :-)
Use PEAR but allow for including the PEAR packages inside your project. All PEAR packages can be separately downloaded from http://pear.php.net/ and can be put anywhere. Depending on convenience and licensing issues you could then package all the required PEAR files with your project or tell users how to download and "install" them.
Reiterating much of what's already been said: http://www.codinghorror.com/blog/archives/001145.html