Internal (built-in) functions
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, get_extension_funcs(),
and dl().
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