__autoload()

(PHP 5, PHP 7)

尝试加载未定义的类

Warning

This feature has beenDEPRECATEDas of PHP 7.2.0. Relying on this featureis highly discouraged.

说明

__autoload(string $class): void

你可以通过定义这个函数来启用类的自动加载。

参数

$class

待加载的类名。

返回值

没有返回值。

参见

• spl_autoload_register() 注册给定的函数作为 __autoload 的实现

It is highly recommended not to use the __autoload() function any more. Now the spl_autoload_register() function is what you should consider.Sorry for the mistake in line 6 of my previous note. And below is the corrected PHP code.
<?php
  if(!function_exists('classAutoLoader')){
    function classAutoLoader($class){
      $class=strtolower($class);
      $classFile=$_SERVER['DOCUMENT_ROOT'].'/include/class/'.$class.'.class.php';
      if(is_file($classFile)&&!class_exists($class)) include $classFile;
    }
  }
  spl_autoload_register('classAutoLoader');
?>

Even I have never been using this function, just a simple example in order to explain it;
./myClass.php
<?php
class myClass {
  public function __construct() {
    echo "myClass init'ed successfuly!!!";
  }
}
?>
./index.php
<?php
// we've writen this code where we need
function __autoload($classname) {
  $filename = "./". $classname .".php";
  include_once($filename);
}
// we've called a class ***
$obj = new myClass();
?>
*** At this line, our "./myClass.php" will be included! This is the magic that we're wondering... And you get this result "myClass init'ed successfuly!!!".
So, if you call a class that named as myClass then a file will be included myClass.php if it exists (if not you get an include error normally). If you call Foo, Foo.php will be included, and so on...
And you don't need some code like this anymore;
<?php
include_once("./myClass.php");
include_once("./myFoo.php");
include_once("./myBar.php");
$obj = new myClass();
$foo = new Foo();
$bar = new Bar();
?>
Your class files will be included "automatically" when you call (init) them without these functions: "include, include_once, require, require_once".

qeremy, your code is incorrect.
<?php
include_once("./myClass.php");
include_once("./myFoo.php");
include_once("./myBar.php");
$obj = new myClass();
$foo = new Foo();
$bar = new Bar();
?>
<?php
$foo = new Foo();
$bar = new Bar();
?>
should be:
<?php
$foo = new myFoo();
$bar = new myBar();
?>

If you can keep file name and class name as same, it will be good programming practice. It helps to __autoload function to load file without checking any condition.
 
function __autoload($class){
 require_once( $class.".php");
}

keyboardSmasher
You may or may not be right as the file name doesn't necessarily have to reflect a class name it contains (but it's usually considered a good practice). It isn't always a straightforward mapping (look at some PHP frameworks and autoload implementations).
File myBar.php may contain the class:
class Bar {
}
or it even contain the class:
class Foo {
}
Best regards

Simple autoload : 
function __autoload($class_name){
      require 'class/' . $class_name . '.php';
}

You should use include() or require() inside __autoload()
instead of include_once() or require_once().
If you reach __autoload(), then you know the file with the class definition has not been loaded yet.
include() and require() are more efficient than include_once() and require_once().

In PHP 7.2, this code will trigger a "
Deprecated: __autoload() is deprecated, use spl_autoload_register() instead in path\to\file.php on line *" error although the spl_autoload_register function really really exists.
<?php
  if(!function_exists('spl_autoload_register')){
    function __autoload($class){
      // blah blah blah
    }
  }
?>

Guys, this document ( i mean __autoload() ) not mentioned one special situation: if you both use __autoload() and spl_autoload_register(), the __autoload() function will never to be called. Although spl_autoload_register() documentation explained why, i decide to wrote this in case some one get confused and waste all day to figure out why.
Here is some code to verify above:
<?php
function __autoload($class) {
}
function my_loader() {
}
function your_loader() {
}
var_dump ( spl_autoload_functions () );
echo '<br/>';
spl_autoload_register ( 'my_loader' );
spl_autoload_register ( 'your_loader' );
var_dump ( spl_autoload_functions () );

<?php
  if(!function_exists('classAutoLoader')){
    function classAutoLoader($class){
      $class=strtolower($class);
      $classFile=$_SERVER['DOCUMENT_ROOT'].'/include/class/'.$class.'.class.php';
      if(is_file($classFile)&&!class_exists($class)) include $classFile;
    }//You have to give the class name and not the class's path for the first parameter in the class_exists() 
  }
  spl_autoload_register('classAutoLoader');
?>

It is highly recommended not to use the __autoload() function any more. Now the spl_autoload_register() function is what you should consider.
<?php
  if(!function_exists('classAutoLoader')){
    function classAutoLoader($class){
      $class=strtolower($class);
      $classFile=$_SERVER['DOCUMENT_ROOT'].'/include/class/'.$class.'.class.php';
      if(is_file($classFile)&&!class_exists($classFile)) include $classFile;
    }
  }
  spl_autoload_register('classAutoLoader');
?>