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浅谈PHP源码二十八：关于类结构和继承
作为面向对象中一个非常关键也非常纠结的特性，我们需要了解一些
在PHP5中，从一开始就有了继承的概念，今天我们从PHP源码出发，了解他是怎么实现的。
在了解类的继承之前，我们需要知道类在PHP源码中是以哪种方式存储的。
找到zend/zend.h 418行：
struct _zend_class_entry {
char type;
char *name;/* 类名 */
zend_uint name_length;/* 类名字符串长度 */
struct _zend_class_entry *parent; /* 父类 */
int refcount; /* 引用计数 */
zend_bool constants_updated;
zend_uint ce_flags;/* 类的访问控制 */
HashTable function_table;/*类的成员函数 */
HashTable default_properties;/*类的默认属性 */
HashTable properties_info;/*类的属性信息 如访问控制等 */
HashTable default_static_members;/*静态成员列表 */
HashTable *static_members;
HashTable constants_table;/* 常量列表 */
const struct _zend_function_entry *builtin_functions; union _zend_function *constructor;/*构造函数*/
union _zend_function *destructor;/*析构函数*/
union _zend_function *clone;/*克隆方法*/ /*魔术方法 */
union _zend_function *__get;
union _zend_function *__set;
union _zend_function *__unset;
union _zend_function *__isset;
union _zend_function *__call;
union _zend_function *__callstatic;
union _zend_function *__tostring;
union _zend_function *serialize_func;
union _zend_function *unserialize_func;
zend_class_iterator_funcs iterator_funcs; /* handlers */
zend_object_value (*create_object)(zend_class_entry *class_type TSRMLS_DC);
zend_object_iterator *(*get_iterator)(zend_class_entry *ce, zval *object, int by_ref TSRMLS_DC);
int (*interface_gets_implemented)(zend_class_entry *iface, zend_class_entry *class_type TSRMLS_DC); /* a class implements this interface */
union _zend_function *(*get_static_method)(zend_class_entry *ce, char* method, int method_len TSRMLS_DC); /* serializer callbacks */
int (*serialize)(zval *object, unsigned char **buffer, zend_uint *buf_len, zend_serialize_data *data TSRMLS_DC);
int (*unserialize)(zval **object, zend_class_entry *ce, const unsigned char *buf, zend_uint buf_len, zend_unserialize_data *data TSRMLS_DC);
zend_class_entry **interfaces;/*类实现的接口 */
zend_uint num_interfaces;/*类实现的接口数 */ /*类所在文件信息 */
char *filename;
zend_uint line_start;
zend_uint line_end;
char *doc_comment;
zend_uint doc_comment_len; struct _zend_module_entry *module;};从PHP自带的SPL中，我们可以看到有类的继承，从这里出发，可以找到我们所需要的实现过程
从手册中选择CachingIterator类作为切入点。
CachingIterator extends IteratorIterator implements OuterIterator , Traversable , Iterator , ArrayAccess , Countable
跟踪过程如下：
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//在ext/spl/spl_iterators.c 3246行，REGISTER_SPL_SUB_CLASS_EX(CachingIterator, IteratorIterator, spl_dual_it_new, spl_funcs_CachingIterator); // ext/spl/spl_functions.h 34行#define REGISTER_SPL_SUB_CLASS_EX(class_name, parent_class_name, obj_ctor, funcs) \
spl_register_sub_class(&spl_ce_ ## class_name, spl_ce_ ## parent_class_name, # class_name, obj_ctor, funcs TSRMLS_CC); // ext/spl/spl_functions.c 56行PHPAPI void spl_register_sub_class(zend_class_entry ** ppce, zend_class_entry * parent_ce, char * class_name, void *obj_ctor, const zend_function_entry * function_list TSRMLS_DC)*ppce = zend_register_internal_class_ex(&ce, parent_ce, NULL TSRMLS_CC); // zend/zend_API.c 2196行ZEND_API zend_class_entry *zend_register_internal_class_ex(zend_class_entry *class_entry, zend_class_entry *parent_ce, char *parent_name TSRMLS_DC)zend_do_inheritance(register_class, parent_ce TSRMLS_CC); // zend/zend_compile.c 2784行
ZEND_API void zend_do_inheritance(zend_class_entry *ce, zend_class_entry *parent_ce TSRMLS_DC){
/* 接口不能继承类 */
if ((ce->ce_flags & ZEND_ACC_INTERFACE)
&& !(parent_ce->ce_flags & ZEND_ACC_INTERFACE)) {
zend_error(E_COMPILE_ERROR, "Interface %s may not inherit from class (%s)", ce->name, parent_ce->name);
} /* final类不能继承 */
if (parent_ce->ce_flags & ZEND_ACC_FINAL_CLASS) {
zend_error(E_COMPILE_ERROR, "Class %s may not inherit from final class (%s)", ce->name, parent_ce->name);
}
ce->parent = parent_ce;
/* 拷贝序列化和反序列化回调函数 */
if (!ce->serialize) {
ce->serialize = parent_ce->serialize;
}
if (!ce->unserialize) {
ce->unserialize = parent_ce->unserialize;
} /* 继承父类的接口 在此处有停下来思考，为何要这么做 */
zend_do_inherit_interfaces(ce, parent_ce TSRMLS_CC); /* 继承父类的属性 */
zend_hash_merge(&ce->default_properties, &parent_ce->default_properties, (void (*)(void *)) zval_add_ref, NULL, sizeof(zval *), 0);
if (parent_ce->type != ce->type) {
/* 用户自定义的类从内部类继承 */
zend_update_class_constants(parent_ce TSRMLS_CC);
zend_hash_apply_with_arguments(CE_STATIC_MEMBERS(parent_ce) TSRMLS_CC, (apply_func_args_t)inherit_static_prop, 1, &ce->default_static_members);
} else {
zend_hash_apply_with_arguments(&parent_ce->default_static_members TSRMLS_CC, (apply_func_args_t)inherit_static_prop, 1, &ce->default_static_members);
}
zend_hash_merge_ex(&ce->properties_info, &parent_ce->properties_info, (copy_ctor_func_t) (ce->type & ZEND_INTERNAL_CLASS ? zend_duplicate_property_info_internal : zend_duplicate_property_info), sizeof(zend_property_info), (merge_checker_func_t) do_inherit_property_access_check, ce); /* 合并常量和成员函数 */
zend_hash_merge(&ce->constants_table, &parent_ce->constants_table, (void (*)(void *)) zval_add_ref, NULL, sizeof(zval *), 0);
zend_hash_merge_ex(&ce->function_table, &parent_ce->function_table, (copy_ctor_func_t) do_inherit_method, sizeof(zend_function), (merge_checker_func_t) do_inherit_method_check, ce);
do_inherit_parent_constructor(ce); if (ce->ce_flags & ZEND_ACC_IMPLICIT_ABSTRACT_CLASS && ce->type == ZEND_INTERNAL_CLASS) {
ce->ce_flags |= ZEND_ACC_EXPLICIT_ABSTRACT_CLASS;
} else if (!(ce->ce_flags & ZEND_ACC_IMPLEMENT_INTERFACES)) {
/* The verification will be done in runtime by ZEND_VERIFY_ABSTRACT_CLASS */
zend_verify_abstract_class(ce TSRMLS_CC);
}}关于上面do_inherit_parent_constructor(ce)
此方法实现魔术方法继承，如果子类中没有相关的魔术方法，则继承父类的对应方法。如下所示的PHP代码为子类没构造函数的情况
class Base {
public function __construct() {
echo 'Base __construct<br />';
}} class Foo extends Base { } $foo = new Foo();如上，会输出：Base __construct
显然继承了父类的构造方法，如果子类有自己的构造方法，并且需要调用父类的构造方法时，需要在子类的构造方法中调用父类的构造方法，PHP不会自动调用。
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