55-Classification of functions. Classification of residential floors. Classification of residential buildings according to the number of floors. The meaning and classification of numbers.

55-Classification of functions

A function is a block of code that can be executed whenever needed. It includes not only user-defined functions, but also library functions implemented in programming languages.

User-Defined Functions

The pseudocode from the manual showing the purpose of the function is shown below:

<code>function foo($arg_1, $arg_2, ..., $arg_n) {
    echo "Example function.\n";
    return $retval;
}
</code>

Any valid PHP code can be written inside a function, even other functions and class definitions.

In PHP 3, functions must be defined before being called. PHP 4 no longer has such conditions. Unless the function is conditionally defined as in the following two examples.

Internal functions

PHP has many standard functions and structures. For example, our common functions such as count, strpos, implode, etc., these are standard functions, and they are provided by standard extensions; For example, we often use isset, empty, eval and other functions, these structures are called language structures. There are also some functions that need to be compiled and enabled with specific PHP extension modules, otherwise they cannot be used. That is, some extensions are optional.

The implementation of standard functions is stored in the ext/standard extension directory.

Anonymous function

Sometimes our code does not need to specify a name for it, but only needs it to complete a specific job. The role of anonymous functions is to expand the use of functions. Before PHP 5.3, function callbacks were passed Method, we only have two options:

• The function name of the string
• The return created using create_function

After PHP5.3, we have one more option – Closure. In terms of implementation, the support for anonymous functions in PHP 5.3 is implemented by using the external variables to be maintained as the "Static attributes" of the Closure object. We will introduce how to implement this in a later chapter.

Variable functions

PHP supports the concept of variable functions. This means that if there are parentheses after a variable name, PHP will look for a function with the same name as the variable's value, and will try to execute it. In addition, this can be used to implement callback functions, function tables, etc. A simple example of a variable function:

<code>$func = 'print_r';
$func('i am print_r function.');
</code>

Variable functions cannot be used in language structures (echo, etc.)

').addClass('pre-numbering').hide(); $(this).addClass('has-numbering').parent().append($numbering); for (i = 1; i ').text(i)); }; $numbering.fadeIn(1700); }); });

The above has introduced the classification of 55-functions, including the classification of numbers and the content of 55. I hope it will be helpful to friends who are interested in PHP tutorials.