A function may be defined using syntax such as the following: ]]> Any valid PHP code may appear inside a function, even other functions and class definitions. In PHP 3, functions must be defined before they are referenced. No such requirement exists since PHP 4. Except when a function is conditionally defined such as shown in the two examples below. When a function is defined in a conditional manner such as the two examples shown. Its definition must be processed prior to being called. ]]> ]]> PHP does not support function overloading, nor is it possible to undefine or redefine previously-declared functions. Function names are case-insensitive, though it is usually good form to call functions as they appear in their declaration. PHP 3 does not support variable numbers of arguments to functions, although default arguments are supported (see Default argument values for more information). Both are supported, as of PHP 4: see Variable-length argument lists and the function references for func_num_args, func_get_arg, and func_get_args for more information. Information may be passed to functions via the argument list, which is a comma-delimited list of expressions. PHP supports passing arguments by value (the default), passing by reference, and default argument values. Variable-length argument lists are supported only in PHP 4 and later; see Variable-length argument lists and the function references for func_num_args, func_get_arg, and func_get_args for more information. A similar effect can be achieved in PHP 3 by passing an array of arguments to a function: ]]> By default, function arguments are passed by value (so that if you change the value of the argument within the function, it does not get changed outside of the function). If you wish to allow a function to modify its arguments, you must pass them by reference. If you want an argument to a function to always be passed by reference, you can prepend an ampersand (&) to the argument name in the function definition: ]]> A function may define C++-style default values for scalar arguments as follows: ]]> The output from the above snippet is: Making a cup of cappuccino. Making a cup of espresso. Also PHP allows you to use arrays and special type NULL as default values, for example: ]]> The default value must be a constant expression, not (for example) a variable, a class member or a function call. Note that when using default arguments, any defaults should be on the right side of any non-default arguments; otherwise, things will not work as expected. Consider the following code snippet: ]]> The output of the above example is: Warning: Missing argument 2 in call to makeyogurt() in /usr/local/etc/httpd/htdocs/php3test/functest.html on line 41 Making a bowl of raspberry . Now, compare the above with this: ]]> The output of this example is: Making a bowl of acidophilus raspberry. As of PHP 5, default values may be passed by reference. PHP 4 and above has support for variable-length argument lists in user-defined functions. This is really quite easy, using the func_num_args, func_get_arg, and func_get_args functions. No special syntax is required, and argument lists may still be explicitly provided with function definitions and will behave as normal. Values are returned by using the optional return statement. Any type may be returned, including lists and objects. This causes the function to end its execution immediately and pass control back to the line from which it was called. See return for more information. ]]> You can't return multiple values from a function, but similar results can be obtained by returning a list. ]]> To return a reference from a function, you have to use the reference operator & in both the function declaration and when assigning the returned value to a variable: ]]> For more information on references, please check out References Explained. PHP supports the concept of variable functions. This means that if a variable name has parentheses appended to it, PHP will look for a function with the same name as whatever the variable evaluates to, and will attempt to execute it. Among other things, this can be used to implement callbacks, function tables, and so forth. Variable functions won't work with language constructs such as echo, print, unset, isset, empty, include, require and the like. You need to use your own wrapper function to utilize any of these constructs as variable functions. \n"; } function bar($arg = '') { echo "In bar(); argument was '$arg'.
\n"; } // This is a wrapper function around echo function echoit($string) { echo $string; } $func = 'foo'; $func(); // This calls foo() $func = 'bar'; $func('test'); // This calls bar() $func = 'echoit'; $func('test'); // This calls echoit() ?> ]]> You can also call an object's method by using the variable functions feature. $name(); // This calls the Bar() method } function Bar() { echo "This is Bar"; } } $foo = new Foo(); $funcname = "Variable"; $foo->$funcname(); // This calls $foo->Variable() ?> ]]> See also call_user_func, variable variables and function_exists. 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.