众所周知，由于历史原因，php之前是使用类名作为构造函数，在php 5中引入的新的构造函数__construct。为了实现向后兼容性，如果php 5在类中找不到 __construct() 函数，它就会尝试寻找旧式的构造函数，也就是和类同名的函数。
因此唯一会产生兼容性问题的情况是：类中已有一个名为 __construct() 的方法，但它却又不是构造函数。有如下一段代码：
<ol class="dp-c"><li class="alt"><span><span><?php </span></span></li><li><span class="keyword">class</span><span> Foo { </span></li><li class="alt"><span> </span></li><li><span> </span><span class="keyword">public</span><span> </span><span class="keyword">function</span><span> Foo() { </span></li><li class="alt"><span> </span></li><li><span> } </span></li><li class="alt"><span> </span></li><li><span> </span><span class="keyword">private</span><span> </span><span class="keyword">function</span><span> __construct() { </span></li><li class="alt"><span> </span></li><li><span> } </span></li><li class="alt"><span>} </span></li><li><span> </span></li><li class="alt"><span class="keyword">new</span><span> Foo(); </span></li><li><span class="keyword">die</span><span>(); </span></li></ol>此时，输出为：
Fatal error: Call to private Foo::__construct() from invalid context
此时，PHP识别出来的构造函数是__construct,因为是private，于是在外部调用出错。好吧，我们从PHP的C源码中查找一下原因吧。从SQL的扩展类中直接查找类的定义开始：
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<ol class="dp-c"><li class="alt"><span><span>spl_iterators.c 3228行 REGISTER_SPL_STD_CLASS_EX(IteratorIterator, spl_dual_it_new, spl_funcs_IteratorIterator); </span></span></li><li><span class="comment">///spl_functions.h 31行 </span><span> </span></li><li class="alt"><span>#define REGISTER_SPL_STD_CLASS_EX(class_name, obj_ctor, funcs) \ </span></li><li><span> spl_register_std_class(&spl_ce_ ## class_name, # class_name, obj_ctor, funcs TSRMLS_CC); </span></li><li class="alt"><span class="comment">//spl_functions.c 41行 </span><span> </span></li><li><span>PHPAPI void spl_register_std_class(zend_class_entry ** ppce, char * class_name, void * obj_ctor, </span><span class="keyword">const</span><span> zend_function_entry * function_list TSRMLS_DC) </span></li><li class="alt"><span> </span></li><li><span class="comment">//spl_functions.c 2235行 </span><span> </span></li><li class="alt"><span>ZEND_API zend_class_entry *zend_register_internal_class(zend_class_entry *orig_class_entry TSRMLS_DC) </span><span class="comment">/* {{{ */</span><span> </span></li><li><span class="comment">//调用do_register_internal_class函数 </span><span> </span></li><li class="alt"><span> </span></li><li><span class="comment">//zend_API.c 2169行 </span><span> </span></li><li class="alt"><span class="keyword">static</span><span> zend_class_entry *do_register_internal_class(zend_class_entry *orig_class_entry, zend_uint ce_flags TSRMLS_DC) </span><span class="comment">/* {{{ */</span><span> </span></li><li><span class="comment">//调用 </span><span> </span></li><li class="alt"><span>zend_register_functions(class_entry, class_entry->builtin_functions, &class_entry->function_table, MODULE_PERSISTENT TSRMLS_CC); </span></li><li><span> </span></li><li class="alt"><span class="comment">//zend_API.c 1795行 </span><span> </span></li><li><span class="comment">/* Look for ctor, dtor, clone </span> </li><li class="alt"><span><span class="comment">* If it's an old-style constructor, store it only if we don't have </span> </span></li><li><span><span class="comment">* a constructor already. </span> </span></li><li class="alt"><span><span class="comment">*/</span><span> </span></span></li><li><span class="keyword">if</span><span> ((fname_len == class_name_len) && !memcmp(lowercase_name, lc_class_name, class_name_len+1) && !ctor) { </span></li><li class="alt"><span> ctor = reg_function; </span></li><li><span>} </span><span class="keyword">else</span><span> </span><span class="keyword">if</span><span> ((fname_len == sizeof(ZEND_CONSTRUCTOR_FUNC_NAME)-1) && !memcmp(lowercase_name, ZEND_CONSTRUCTOR_FUNC_NAME, sizeof(ZEND_CONSTRUCTOR_FUNC_NAME))) { </span></li><li class="alt"><span> ctor = reg_function; </span></li><li><span>} </span></li><li class="alt"><span> </span></li><li><span>scope->constructor = ctor; </span><span class="comment">//在1961行 确认构造函数 </span><span> </span></li></ol>以上代码为PHP 5.3.0版本
从以上跟踪流程来看，程序在注册所有函数时，如果存在__construct(即ZEND_CONSTRUCTOR_FUNC_NAME)时，会覆盖class_name(类名)的构造函数，使其作为常规的成员函数存在。如下所示代码：
<ol class="dp-xml"><li class="alt"><span><span class="tag"><?</span><span class="tag-name">php</span><span> </span></span></li><li><span>class Foo { </span></li><li class="alt"><span> </span></li><li><span> public function Foo() { </span></li><li class="alt"><span> echo 'Foo'; </span></li><li><span> } </span></li><li class="alt"><span> </span></li><li><span> public function __construct() { </span></li><li class="alt"><span> echo '__construct'; </span></li><li><span> } </span></li><li class="alt"><span>} </span></li><li><span> </span></li><li class="alt"><span>$</span><span class="attribute">foo</span><span> = </span><span class="attribute-value">new</span><span> Foo(); </span></li><li><span>$foo-</span><span class="tag">></span><span>Foo(); </span></li></ol>对于在前面的示例中的报错，我们可以在zend/zend_object_handlers.c 1057行ZEND_API union _zend_function *zend_std_get_constructor(zval *object TSRMLS_DC)找到出处。
