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;
?>