class_alias() gives you the ability to do conditional imports.

Whereas the following will not work:

<?php
namespace Component;

if (version_compare(PHP_VERSION, '5.4.0', 'gte')) {
    use My\ArrayObject;
} else {
    use ArrayObject;
}

class Container extends ArrayObject
{
}
?>

the following, using class_alias, will:

<?php
namespace Component;

if (version_compare(PHP_VERSION, '5.4.0', 'lt')) {
    class_alias('My\ArrayObject', 'Component\ArrayObject');
} else {
    class_alias('ArrayObject', 'Component\ArrayObject');
}

class Container extends ArrayObject
{
}
?>

The semantics are slightly different (I'm now indicating that Container extends from an ArrayObject implementation in the same namespace), but the overall idea is the same: conditional imports.

It also works with Traits!

<?php 
trait Foo {}
class_alias("Foo","Bar");
echo trait_exists("Bar") ? 'yes' : 'no'; 
?>
//yes

If you defined the class 'original' in a namespace, you will have to specify the namespace(s), too:
<?php
namespace ns1\ns2\ns3;

class A {}

class_alias('ns1\ns2\ns3\A', 'B');
/* or if you want B to exist in ns1\ns2\ns3 */
class_alias('ns1\ns2\ns3\A', 'ns1\ns2\ns3\B');
?>

class_alias also works for interfaces!

<?php
interface foo {}
class_alias('foo', 'bar');
echo interface_exists('bar') ? 'yes!' : 'no'; // prints yes!
?>

You can add a alias inside a class :

<?php

class foo{

    function __construct(){
        
        echo('yes!');
    
    }
}

class bar {

    function __construct(){
        
        class_alias('foo', 'fooAlias');
    
    }
    
    function test(){
        
        new fooAlias;
        
    }

}

$bar=new bar;

$bar->test(); // yes!

class_alias() creates aliases only for user defined classes, not for classes supplied by PHP (PHP will show the warning "First argument of class_alias() must be a name of user defined class"). To create aliases for every kind of classes, use namespaces:

<?php

// Does not work
class_alias("ZipArchive", "myZip");

// Creates class alias "myZip" of class "ZipArchive"
use \ZipArchive as myZip;

?>

The alias really is an alias for the existing class. It's not a new class of any kind - whether by inheritance or otherwise; it doesn't just look and behave exactly like the existing class; it really is the same class.

<?php

class foo
{
    public static $count = 0;
}

class_alias('foo', 'bar');

bar::$count++;

echo foo::$count; // Output: 1

echo get_class(new Bar); // Output: foo
?>
Note in the last line there that aliases are just as case-insensitive as "genuine" class names.

At first, you might wonder that:
<?php class A {}; class_alias('A', 'B'); ?>

is equivalent to:
<?php class A {}; class B extends A {}; ?>

class_alias is NOT equivalent to class extending! Private methods/properties are unseen in child classes, but in alias classes they are.

Something to note,

If the $original class has not yet been defined or loaded, the auto loader will be invoked in order to try and load it.

If the class for which you are trying to create an alias does not exist, or can not be loaded with the auto loader, you will generate a PHP Warning.

Check is alias:
<?php
/**
 * @param string $class Class name
 * @return bool
 */
function is_alias(string $class): bool
{
    return $class !== (new ReflectionClass($class))->name;
}
?>
Get class aliases:
<?php
/**
 * @param string $class Class name
 * @param bool $throw Throw exception at error?
 * @return string[]|null Aliases or null at error in silent mode
 * @throws InvalidArgumentException Class not exists or it's alias
 */
function get_class_aliases(string $class, bool $throw = false): ?array
{
    /**
     * @var array An array of defined classes: keys - classes, values - aliases
     */
    static $classes = [];
    // check: class exists
    if (! class_exists($class, true)) {
        if ($throw) {
            throw new InvalidArgumentException('Class ' . $class . ' not exists');
        }
        return null;
    }
    // refresh list
    $newClasses = array_diff(get_declared_classes(), array_keys($classes));
    if ($newClasses) {
        $abc = range('a', 'z');
        foreach ($newClasses as $newClass) {
            // fast check first char: class_alias() convert alias to lower case
            if (in_array($newClass[0], $abc, true)) {
                $realClass = (new ReflectionClass($newClass))->getName();
                $classes[$newClass] = $newClass !== $realClass ? $realClass : null;
            } else {
                $classes[$newClass] = null;
            }
        }
        unset($abc, $newClasses);
    }
    // check: is alias?
    if (! empty($classes[$class])) {
        if ($throw) {
            throw new InvalidArgumentException($class . ' is alias for class ' . $classes[$class]);
        }
        return null;
    }
    // find aliases
    return array_keys($classes, $class, true);
}
?>
Usage:
<?php
class Foo {}
class_alias('Foo', 'Bar');
class_alias('Bar', 'Baz');
$aliases = get_class_aliases('Foo', true); // ['bar', 'baz']
?>

Note: this function set alias for user classes, you can't use something like this
<?php
class_alias('ArrayObject', 'ArrObj');
?>

At first, you might wonder that:
<?php class A {}; class_alias('A', 'B'); ?>

is equivalent to:
<?php class A {}; class B extends A {}; ?>

BUT when derivation creates a new class name - that means, you can then instantiate a new kind of objects - aliasing is just what it says: a synonym, so objects instantiated with the aliased name are of the exact same kind of objects instantiated with the non-aliased name.

See this code for example:
<?php
class A {};
class B1 extends A {};
class_alias('A', 'B2');

$b1 = new B1; echo get_class($b1); // prints B1
$b2 = new B2; echo get_class($b2); // prints A !
?>

Doesn't work with coupled classes when used along with autoloading.

For example, in these classes where each class is autoloaded in a separate class file:

Foo.php:

<?php
interface Foo{
  public function fx(Bar $bar);
}
?>

Bar2.php:

<?php
class Bar2 implements Foo{
  public function fx(Bar2 $bar){
    // some implementation code here
  }
}
?>

Bar.php:

<?php
class_alias("Bar2", "Bar");
?>

When used with an autoloader like this:

<?php
spl_autoload_register(function($class){
  require($class . ".php");
});
new Bar;
?>

Results in fatal error:

    Declaration of Bar2::fx(Bar2 $bar) must be compatible with Foo::fx(Bar $bar) in ~/Bar2.php on line 2

Doesn't work with coupled classes when used along with autoloading.

For example, in these classes where each class is autoloaded in a separate class file:

Foo.php:

<?php
interface Foo{
  public function fx(Bar $bar);
}
?>

Bar2.php:

<?php
class Bar2 implements Foo{
  public function fx(Bar2 $bar){
    // some implementation code here
  }
}
?>

Bar.php:

<?php
class_alias("Bar2", "Bar");
?>

When used with an autoloader like this:

<?php
spl_autoload_register(function($class){
  require($class . ".php");
});
new Bar;
?>

Results in fatal error:

    Declaration of Bar2::fx(Bar2 $bar) must be compatible with Foo::fx(Bar $bar) in ~/Bar2.php on line 2