PHP custom function official documentation

A function can be defined by the following syntax:

Example #1 Pseudocode showing the purpose of the function

<span><span><?php<br></span><span>function </span><span>foo</span><span>(</span><span>$arg_1</span><span> , </span> <span>$arg_2</span><span>, </span><span>/* ..., */ </span><span>$arg_n</span><span>)<br>{<br> echo </span><span>"Example function.n"</span><span>;<br> return </span><span>$ret val</span><span>;<br>}<br> </span><span>?></span></span>

Any valid PHP code may appear inside a function, even other function and class definitions.

Function names have the same naming rules as other identifiers in PHP. Valid function names begin with a letter or underscore, followed by letters, numbers, or underscores. It can be expressed as a regular expression: [a-zA-Z_x7f-xff][a-zA-Z0-9_x7f-xff]*.

Tip

Function does not need to be defined before calling, unlessin the following two examples when the function is conditionally defined.

When a function is conditionally defined, it must be defined before calling the function .

Example #2 Conditional function

<span><span><?php<br>$makefoo </span><span>= </span><span>true</span><span>;<br></span><span>/* The foo() function cannot be called here, <br> because it It doesn't exist yet, but the bar() function can be called. */<br></span><span>bar</span><span>();<br>if (</span><span>$makefoo</span><span>) {<br> function </span><span>foo</span><span>()<br> {<br> echo </span><span>"I don't exist until program execution reaches me.n "</span><span>;<br> }<br>}<br></span><span>/* It is now safe to call function foo(), <br> because $makefoo is true */<br></span><span>if (</span><span>$makefoo</span><span>) </span><span>foo</span> <span>( );<br>function </span><span>bar</span><span>()<br>{<br> echo </span><span>"I exist immediately upon program start.n"</span><span>;<br>}<br></span><span>?></span></span>

Example #3 Function Function in

<span><span><?php<br></span><span>function </span><span>foo</span><span>()<br>{<br> function </span><span>bar</span><span>()<br> {<br> echo </span><span>"I don't exist until foo( ) is called.n"</span><span>;<br> }<br>}<br></span><span>/* You can’t call the bar() function now because it doesn’t exist yet */<br></span><span>foo</span><span>();<br></span><span>/* You can now The bar() function is called, because the execution of the foo() function <br> makes the bar() function a defined function */<br></span><span>bar</span><span>();<br></span><span>?></span></span>

All functions and classes in PHP have global scope and can be defined within a function and called outside it, and vice versa.

PHP does not support function overloading, and it is not possible to undefine or redefine declared functions.

Note: Function names are case-insensitive, but when calling a function, it is a good habit to use the same form as when it was defined.

You can call recursive functions in PHP.

Example #4 Recursive function

<span><span><?php<br></span><span>function </span><span>recursion</span><span>(</span><span>$a</span><span>)<br>{<br>         if ( </span><span>$a </span><span>< </span><span> 20. </span><span>1<br></span>);<span>  }</span>}<span></span><span>? ></span><span><br></span><span></span><span></span>Note: <span>But avoid recursive function/method calls beyond 100-200 levels as it may crash the stack and terminate the current script. Infinite recursion can be considered a programming error. </span><span></span><span></span>Copyright Statement: This article is an original article by the blogger and may not be reproduced without the blogger's permission. <span><br> <br> The above introduces the official documentation of PHP custom functions, including relevant content. I hope it will be helpful to friends who are interested in PHP tutorials. <br> </span> <span></span></span>