Functions
PHP has hundreds of base functions and thousands more from extensions. Functions are not first-class functions and can only be referenced by their name prior to PHP version 5.3.0, whereas PHP 5.3.0 introduces closures. User-defined functions can be created at any time and without being prototyped. Functions can be defined inside code blocks, permitting a run-time decision as to whether or not a function should be defined. There is no concept of local functions. Function calls must use parentheses with the exception of zero argument class constructor functions called with the PHP new operator, where parentheses are optional.
An example function definition is the following:
Prior to version 5.3, PHP only supports quasi-anonymous functions through the create_function function. These are not true anonymous functions because anonymous functions are nameless but functions can only be referenced by name in PHP. As of version 5.3, PHP supports true anonymous functions.
Function calls may be made via variables, where the value of a variable contains the name of the function to call. This is illustrated in the following example:
PHP does not support named parameters or parameter skipping. Some core PHP developers have publicly expressed disappointment with this decision. Others have suggested workarounds for this limitation.
Read more about this topic: PHP Syntax And Semantics
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