<?php
session_start();
$session['name'] = $name;

static void php_session_save_current_state(TSRMLS_D) /* {{{ */
{
        int ret = FAILURE;
        
        IF_SESSION_VARS() {
                //如果存在Session数组
                if (PS(bug_compat) && !PG(register_globals)) {
                        HashTable *ht = Z_ARRVAL_P(PS(http_session_vars));
                        HashPosition pos;
                        zval **val;
                        int do_warn = 0;
                        
                        zend_hash_internal_pointer_reset_ex(ht, &pos);
                        
                        while (zend_hash_get_current_data_ex(ht, (void **) &val, &pos) != FAILURE) {
                                if (Z_TYPE_PP(val) == IS_NULL) { //变量为null
                                        if (migrate_global(ht, &pos TSRMLS_CC)) {//当前符号表中有同名变量
                                                do_warn = 1;
                                        }
                                }
                                zend_hash_move_forward_ex(ht, &pos);
                        }
                        
                        if (do_warn && PS(bug_compat_warn)) {
                                php_error_docref(NULL TSRMLS_CC, E_WARNING, "Your script possibly relies on a session side-effect which existed until PHP 
                                4.2.3. Please be advised that the session extension does not consider global variables as a source of data, unless register_globals is 
                                enabled. You can disable this functionality and this warning by setting session.bug_compat_42 or session.bug_compat_warn to off, 
                                respectively");
                                //后面省略	
                                

<?php
session_start(); 
$a = 'laruence' ; //在当前符号表插入一个同名变量
$_SESSION['a'] = null; //同名session变量,并且值为null
?>