class_alias.php

<?php
namespace {
    class A {}

    var_dump(class_alias('A', 'B'));            // => bool(true)
    $reflector = new \ReflectionClass('B');
    var_dump($reflector->getName());            // => string(1) "A"
}

namespace jubianchi\Alias {
    class A {}

    var_dump(class_alias('\\jubianchi\\Alias\\A', 'C'));                        // => bool(true)
    $reflector = new \ReflectionClass('C');
    var_dump($reflector->getName());                                            // => jubianchi\Alias\A

                                                   
    var_dump(class_alias('\\jubianchi\\Alias\\A', '\\jubianchi\\Alias\\B'));    // => bool(true)
                                                // ^ the leading anti-slashes seem to make the alias fail
                                                // but the class_alias call returns true as if everything went fine
    try {
        $reflector = new \ReflectionClass('\\jubianchi\\Alias\\B');
        var_dump($reflector->getName());                                        // Expected => jubianchi\Alias\A
    } catch(\Exception $e) {
        var_dump(get_class($e) . ': ' . $e->getMessage());                      // Got => ReflectionException: Class \jubianchi\Alias\B does not exist
    }


    var_dump(class_alias('\\jubianchi\\Alias\\A', 'jubianchi\\Alias\\B'));      // => bool(true)
    $reflector = new \ReflectionClass('\\jubianchi\\Alias\\B');
    var_dump($reflector->getName());                                            // => jubianchi\Alias\A
}

output

$ php class_alias.php
bool(true)
string(1) "A"
bool(true)
string(17) "jubianchi\Alias\A"
bool(true)
string(60) "ReflectionException: Class \jubianchi\Alias\B does not exist"
bool(true)
string(17) "jubianchi\Alias\A"

-- Expected --
bool(true)
string(1) "A"
bool(true)
string(17) "jubianchi\Alias\A"
bool(true)
string(17) "jubianchi\Alias\A"
bool(true)
string(17) "jubianchi\Alias\A"

-- Or expected --
bool(true)
string(1) "A"
bool(true)
string(17) "jubianchi\Alias\A"
bool(false)
string(60) "ReflectionException: Class \jubianchi\Alias\B does not exist"
bool(true)
string(17) "jubianchi\Alias\A"