長久以來，在php中刪除cookie的時候，都是使用
bool setcookie ( string $name [, string $value [, int $expire = 0 [, string $path [, string $domain [, bool $secure = false [, bool $httponly = false ]]]]]] )
$value 隨便寫， $expire設置為一個已經過去的時間即可。
官方文檔中也是這樣寫的：
Example #2 setcookie() delete example
When deleting a cookie you should assure that the expiration date is in the past, to trigger the removal mechanism in your browser. Examples follow how to delete cookies sent in previous example:
<?php
// set the expiration date to one hour ago
setcookie ("TestCookie", "", time() - 3600);
setcookie ("TestCookie", "", time() - 3600, "/~rasmus/", "example.com", 1);
?>
今天遇到一件奇怪的事， 在setcookie的時候，傳了一個空字符串給$value，結果竟然是此cookie被刪除了…
$name = "post_url";
$value = "";
setcookie($name, $value, time()+60*60*3, "/" );
delete_cookie
相當不解。
去翻php 5.4.13 的源碼：
ext/standard/head.c
173 PHP_FUNCTION(setcookie)
174 {
175 char *name, *value = NULL, *path = NULL, *domain = NULL;
176 long expires = 0;
177 zend_bool secure = 0, httponly = 0;
178 int name_len, value_len = 0, path_len = 0, domain_len = 0;
179
180 if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "s|slssbb", &name,
181 &name_len, &value, &value_len, &expires, &path,
182 &path_len, &domain, &domain_len, &secure, &httponly) == FAILURE) {
183 return;
184 }
185
186 if (php_setcookie(name, name_len, value, value_len, expires, path, path_len, domain, domain_len, secure, 1, httponly TSRMLS_CC) == SUCCESS) {
187 RETVAL_TRUE;
188 } else {
189 RETVAL_FALSE;
190 }
191 }
76 PHPAPI int php_setcookie(char *name, int name_len, char *value, int value_len, time_t expires, char *path, int path_len, char *domain, int domain_len, int secure, int url_encode, int httponly TSRMLS_DC)
77 {
78 char *cookie, *encoded_value = NULL;
79 int len=sizeof("Set-Cookie: ");
80 char *dt;
81 sapi_header_line ctr = {0};
82 int result;
83
84 if (name && strpbrk(name, "=,; \t\r\n\013\014") != NULL) { /* man isspace for \013 and \014 */
85 zend_error( E_WARNING, "Cookie names cannot contain any of the following '=,; \\t\\r\\n\\013\\014'" );
86 return FAILURE;
87 }
88
89 if (!url_encode && value && strpbrk(value, ",; \t\r\n\013\014") != NULL) { /* man isspace for \013 and \014 */
90 zend_error( E_WARNING, "Cookie values cannot contain any of the following ',; \\t\\r\\n\\013\\014'" );
91 return FAILURE;
92 }
93
94 len += name_len;
95 if (value && url_encode) {
96 int encoded_value_len;
97
98 encoded_value = php_url_encode(value, value_len, &encoded_value_len);
99 len += encoded_value_len;
100 } else if ( value ) {
101 encoded_value = estrdup(value);
102 len += value_len;
103 }
104 if (path) {
105 len += path_len;
106 }
107 if (domain) {
108 len += domain_len;
109 }
110
111 cookie = emalloc(len + 100);
112
113 if (value && value_len == 0) {
114 /*
115 * MSIE doesn't delete a cookie when you set it to a null value
116 * so in order to force cookies to be deleted, even on MSIE, we
117 * pick an expiry date in the past
118 */
119 dt = php_format_date("D, d-M-Y H:i:s T", sizeof("D, d-M-Y H:i:s T")-1, 1, 0 TSRMLS_CC);
120 snprintf(cookie, len + 100, "Set-Cookie: %s=deleted; expires=%s", name, dt);
121 efree(dt);
122 } else {
123 snprintf(cookie, len + 100, "Set-Cookie: %s=%s", name, value ? encoded_value : "");
124 if (expires > 0) {
125 const char *p;
126 strlcat(cookie, "; expires=", len + 100);
127 dt = php_format_date("D, d-M-Y H:i:s T", sizeof("D, d-M-Y H:i:s T")-1, expires, 0 TSRMLS_CC);
128 /* check to make sure that the year does not exceed 4 digits in length */
129 p = zend_memrchr(dt, '-', strlen(dt));
130 if (!p || *(p + 5) != ' ') {
參數中的value在C語言中的類型是char * ， 還有一個 value_len標明了它的長度。
如果value_len為0的話，就寫了下面的cookie:
值為”deleted”, 過期時間為 Thu, 01-Jan-1970 08:00:01 CST 或者說是 Thu, 01-Jan-1970 00:00:01 GMT
看來setcookie($name, “”) 確實可以刪除這個cookie了…
同理，在php中，strval(NULL) === “” , 所以 setcookie($name, NULL) 也就相當於 setcookie($name, “”)，同樣可以刪除此cookie.
另外，比較好奇的是：
if (value && value_len == 0) {
}
else {
}
else 中包含了 value 為null 的情況, 這是一種什麼樣的情況呢？
