Functions and methods should be named using the "studly caps" style (also referred to as "bumpy case" or "camel caps"). Functions should in addition have the package name as a prefix, to avoid name collisions between packages. The initial letter of the name (after the prefix) is lowercase, and each letter that starts a new "word" is capitalized. Some examples:
Private class members (meaning class members that are intented to be used only from within the same class in which they are declared; PHP does not yet support truly-enforceable private namespaces) are preceeded by a single underscore. For example:
Constants should always be all-uppercase, with underscores to seperate words. Prefix constant names with the uppercased name of the class/package they are used in. For example, the constants used by the DB:: package all begin with "DB_".
If your package needs to define global variables, their name should start with a single underscore followed by the package name and another underscore. For example, the PEAR package uses a global variable called $_PEAR_destructor_object_list.