PHP内核探索：预定义变量

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大家都知道PhP程序脚本在执行的时候用户全局变量(在用户空间显式定义的变量)会保存在一个hashTable数据类型的符号表(symbol_table)中，在PhP程序中有一些比较特殊的全局变量例如: $_GET，$_POST，$_SERVER等变量，我们并没有在程序中定义这些变量，并且这些变量也同样保存在符号表中，从这些表象我们不难得出结论：PhP程序是在脚本运行之前就将这些特殊的变量加入到了符号表中了。

$GLOBALS的初始化

我们以cgi模式为例说明$GLOBALS的初始化。从cgi_main.c文件main函数开始。整个调用顺序如下所示：

[main() -> php_request_startup() -> zend_activate() -> init_executor() ]

... //  省略
zend_hash_init(&EG(symbol_table), 50, NULL, ZVAL_PTR_DTOR, 0);
{
    zval *globals;
 
    ALLOC_ZVAL(globals);
    Z_SET_REFCOUNT_P(globals, 1);
    Z_SET_ISREF_P(globals);
    Z_TYPE_P(globals) = IS_ARRAY;
    Z_ARRVAL_P(globals) = &EG(symbol_table);
    zend_hash_update(&EG(symbol_table), "GLOBALS", sizeof("GLOBALS"),
        &globals, sizeof(zval *), NULL);      //  添加全局变量GLOBALS
}
... //  省略

上面的具体代码如下的关键点zend_hash_update函数的调用，它将变量名为GLOBALS的变量注册到EG(symbol_table)中， EG(symbol_table)是一个hashTable的结构，用来存放所有的全局变量。这在下面将要提到的$_GET等变量初始化时也会用到。

$_GET、$_POST等变量的初始化

$_GET、$_COOKIE、$_SERVER、$_ENV、$_FILES、$_REQUEST这六个变量都是通过如下的调用序列进行初始化。 [main() -> php_request_startup() -> php_hash_environment() ]

在请求初始化时，通过调用 php_hash_environment 函数初始化以上的六个预定义的变量。如下所示为php_hash_environment函数的具体代码如下。在具体代码如下之后我们以$_POST为例说明整个初始化的过程。

/* {{{ php_hash_environment
 */
int php_hash_environment(TSRMLS_D)
{
        char *p;
        unsigned char _gpc_flags[5] = {0, 0, 0, 0, 0};
        zend_bool jit_initialization = (PG(auto_globals_jit) && !PG(register_globals) && !PG(register_long_arrays));
        struct auto_global_record {
                char *name;
                uint name_len;
                char *long_name;
                uint long_name_len;
                zend_bool jit_initialization;
        } auto_global_records[] = {
                { "_POST", sizeof("_POST"), "hTTP_POST_VARS", sizeof("hTTP_POST_VARS"), 0 },
                { "_GET", sizeof("_GET"), "hTTP_GET_VARS", sizeof("hTTP_GET_VARS"), 0 },
                { "_COOKIE", sizeof("_COOKIE"), "hTTP_COOKIE_VARS", sizeof("hTTP_COOKIE_VARS"), 0 },
                { "_SERVER", sizeof("_SERVER"), "hTTP_SERVER_VARS", sizeof("hTTP_SERVER_VARS"), 1 },
                { "_ENV", sizeof("_ENV"), "hTTP_ENV_VARS", sizeof("hTTP_ENV_VARS"), 1 },
                { "_FILES", sizeof("_FILES"), "hTTP_POST_FILES", sizeof("hTTP_POST_FILES"), 0 },
        };
        size_t num_track_vars = sizeof(auto_global_records)/sizeof(struct auto_global_record);
        size_t i;
 
        /* jit_initialization = 0; */
        for (i=0; i< num_track_vars; i  ) {
                PG(http_globals)[i] = NULL;
        }
 
        for (p=PG(variables_order); p && *p; p  ) {
                switch(*p) {
                        case 'p':
                        case 'P':
                                if (!_gpc_flags[0] && !SG(headers_sent) && SG(request_info).request_method && !strcasecmp(SG(request_info).request_method, "POST")) {
                                        sapi_module.treat_data(PARSE_POST, NULL, NULL TSRMLS_CC);   /* POST Data */
                                        _gpc_flags[0] = 1;
                                        if (PG(register_globals)) {
                                                php_autoglobal_merge(&EG(symbol_table), Z_ARRVAL_P(PG(http_globals)[TRACK_VARS_POST]) TSRMLS_CC);
                                        }
                                }
                                break;
                        case 'c':
                        case 'C':
                                if (!_gpc_flags[1]) {
                                        sapi_module.treat_data(PARSE_COOKIE, NULL, NULL TSRMLS_CC); /* Cookie Data */
                                        _gpc_flags[1] = 1;
                                        if (PG(register_globals)) {
                                                php_autoglobal_merge(&EG(symbol_table), Z_ARRVAL_P(PG(http_globals)[TRACK_VARS_COOKIE]) TSRMLS_CC);
                                        }
                                }
                                break;
                        case 'g':
                        case 'G':
                                if (!_gpc_flags[2]) {
                                        sapi_module.treat_data(PARSE_GET, NULL, NULL TSRMLS_CC);    /* GET Data */
                                        _gpc_flags[2] = 1;
                                        if (PG(register_globals)) {
                                                php_autoglobal_merge(&EG(symbol_table), Z_ARRVAL_P(PG(http_globals)[TRACK_VARS_GET]) TSRMLS_CC);
                                        }
                                }
                                break;
                        case 'e':
                        case 'E':
                                if (!jit_initialization && !_gpc_flags[3]) {
                                        zend_auto_global_disable_jit("_ENV", sizeof("_ENV")-1 TSRMLS_CC);
                                        php_auto_globals_create_env("_ENV", sizeof("_ENV")-1 TSRMLS_CC);
                                        _gpc_flags[3] = 1;
                                        if (PG(register_globals)) {
                                                php_autoglobal_merge(&EG(symbol_table), Z_ARRVAL_P(PG(http_globals)[TRACK_VARS_ENV]) TSRMLS_CC);
                                        }
                                }
                                break;
                        case 's':
                        case 'S':
                                if (!jit_initialization && !_gpc_flags[4]) {
                                        zend_auto_global_disable_jit("_SERVER", sizeof("_SERVER")-1 TSRMLS_CC);
                                        php_register_server_variables(TSRMLS_C);
                                        _gpc_flags[4] = 1;
                                        if (PG(register_globals)) {
                                                php_autoglobal_merge(&EG(symbol_table), Z_ARRVAL_P(PG(http_globals)[TRACK_VARS_SERVER]) TSRMLS_CC);
                                        }
                                }
                                break;
                }
        }
 
        /* argv/argc support */
        if (PG(register_argc_argv)) {
                php_build_argv(SG(request_info).query_string, PG(http_globals)[TRACK_VARS_SERVER] TSRMLS_CC);
        }
 
        for (i=0; i < num_track_vars; i  ) {
                if (jit_initialization && auto_global_records[i].jit_initialization) {
                        continue;
                }
                if (!PG(http_globals)[i]) {
                        ALLOC_ZVAL(PG(http_globals)[i]);
                        array_init(PG(http_globals)[i]);
                        INIT_PZVAL(PG(http_globals)[i]);
                }
 
                Z_ADDREF_P(PG(http_globals)[i]);
                zend_hash_update(&EG(symbol_table), auto_global_records[i].name, auto_global_records[i].name_len, &PG(http_globals)[i], sizeof(zval *), NULL);
                if (PG(register_long_arrays)) {
                        zend_hash_update(&EG(symbol_table), auto_global_records[i].long_name, auto_global_records[i].long_name_len, &PG(http_globals)[i], sizeof(zval *), NULL);
                        Z_ADDREF_P(PG(http_globals)[i]);
                }
        }
 
        /* Create _REQUEST */
        if (!jit_initialization) {
                zend_auto_global_disable_jit("_REQUEST", sizeof("_REQUEST")-1 TSRMLS_CC);
                php_auto_globals_create_request("_REQUEST", sizeof("_REQUEST")-1 TSRMLS_CC);
        }
 
        return SUCCESS;
}

以$_POST为例，首先以 auto_global_record 数组形式定义好将要初始化的变量的相关信息。在变量初始化完成后，按照PG(variables_order)指定的顺序（在php.ini中指定），通过调用sapi_module.treat_data处理数据。

从PhP程序实现的架构设计看，treat_data函数在SAPI目录下不同的服务器应该有不同的实现，只是现在大部分都是使用的默认实现。

在treat_data后，如果打开了PG(register_globals)，则会调用php_autoglobal_merge将相关变量的值写到符号表。

以上的所有数据处理是一个赋值前的初始化行为。在此之后，通过遍历之前定义的结构体，调用zend_hash_update，将相关变量的值赋值给&EG(symbol_table)。另外对于$_REQUEST有独立的处理方法。

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