PHP comes standard with many functions and constructs. There are also
functions that require specific PHP extensions compiled in otherwise
you'll get fatal "undefined function" errors. For example, to use
image functions such as
imagecreatetruecolor(), you'll need your PHP compiled
with GD support. Or, to use mysql_connect() you'll
need your PHP compiled in with MySQL
support. There are many core functions that are included in every
version of PHP like the string and
variable functions. A call
to phpinfo() or
get_loaded_extensions() will show you which
extensions are loaded into your PHP. Also note that many extensions are
enabled by default and that the PHP manual is split up by extension.
See the configuration,
installation, and individual
extension chapters, for information on how to setup your PHP.
Reading and understanding a function's prototype is explained within the
manual section titled
how to read a function definition.
It's important to realize what a function returns or if a function works
directly on a passed in value. For example,
str_replace() will return the modified string while
usort() works on the actual passed in variable
itself. Each manual page also has specific information for each
function like information on function parameters, behavior changes,
return values for both success and failure, and availability information.
Knowing these important (yet often subtle) differences is crucial for
writing correct PHP code.
See also function_exists(),
the function reference,