本函数是实验性的。本函数的行为，包括函数名称以及其它任何关于本函数的文档可能会在没有通知的情况下随 PHP 以后的发布而改变。使用本函数风险自担。
The function parses the XML document in str and returns a tree PHP objects as the parsed document. This function is isolated from the other functions, which means you cannot access the tree with any of the other functions. Modifying it, for example by adding nodes, makes no sense since there is currently no way to dump it as an XML file. However this function may be valuable if you want to read a file and investigate the content.