这里是生成id的代码:
Session.c
具体来说，是
php_session_create_id 函数:
PHPAPI char *php_session_create_id(PS_CREATE_SID_ARGS)
{
PHP_MD5_CTX md5_context;
PHP_SHA1_CTX sha1_context;
#if defined(HAVE_HASH_EXT) && !defined(COMPILE_DL_HASH)
void *hash_context = NULL;
#endif
unsigned char *digest;
int digest_len;
int j;
char *buf, *outid;
struct timeval tv;
zval **array;
zval **token;
char *remote_addr = NULL;
gettimeofday(&tv, NULL);
if (zend_hash_find(&EG(symbol_table), "_SERVER", sizeof("_SERVER"), (void **) &array) == SUCCESS &&
Z_TYPE_PP(array) == IS_ARRAY &&
zend_hash_find(Z_ARRVAL_PP(array), "REMOTE_ADDR", sizeof("REMOTE_ADDR"), (void **) &token) == SUCCESS
) {
remote_addr = Z_STRVAL_PP(token);
}
spprintf(&buf, 0, "%.15s%ld%ld%0.8F", remote_addr ? remote_addr : "", tv.tv_sec, (long int)tv.tv_usec, php_combined_lcg(TSRMLS_C) * 10);
switch (PS(hash_func)) {
case PS_HASH_FUNC_MD5:
PHP_MD5Init(&md5_context);
PHP_MD5Update(&md5_context, (unsigned char *) buf, strlen(buf));
digest_len = 16;
break;
case PS_HASH_FUNC_SHA1:
PHP_SHA1Init(&sha1_context);
PHP_SHA1Update(&sha1_context, (unsigned char *) buf, strlen(buf));
digest_len = 20;
break;
#if defined(HAVE_HASH_EXT) && !defined(COMPILE_DL_HASH)
case PS_HASH_FUNC_OTHER:
if (!PS(hash_ops)) {
php_error_docref(NULL TSRMLS_CC, E_ERROR, "Invalid session hash function");
efree(buf);
return NULL;
}
hash_context = emalloc(PS(hash_ops)->context_size);
PS(hash_ops)->hash_init(hash_context);
PS(hash_ops)->hash_update(hash_context, (unsigned char *) buf, strlen(buf));
digest_len = PS(hash_ops)->digest_size;
break;
#endif
default:
php_error_docref(NULL TSRMLS_CC, E_ERROR, "Invalid session hash function");
efree(buf);
return NULL;
}
efree(buf);
if (PS(entropy_length) > 0) {
#ifdef PHP_WIN32
unsigned char rbuf[2048];
size_t toread = PS(entropy_length);
if (php_win32_get_random_bytes(rbuf, MIN(toread, sizeof(rbuf))) == SUCCESS){
switch (PS(hash_func)) {
case PS_HASH_FUNC_MD5:
PHP_MD5Update(&md5_context, rbuf, toread);
break;
case PS_HASH_FUNC_SHA1:
PHP_SHA1Update(&sha1_context, rbuf, toread);
break;
# if defined(HAVE_HASH_EXT) && !defined(COMPILE_DL_HASH)
case PS_HASH_FUNC_OTHER:
PS(hash_ops)->hash_update(hash_context, rbuf, toread);
break;
# endif
}
}
#else
int fd;
fd = VCWD_OPEN(PS(entropy_file), O_RDONLY);
if (fd >= 0) {
unsigned char rbuf[2048];
int n;
int to_read = PS(entropy_length);
while (to_read > 0) {
n = read(fd, rbuf, MIN(to_read, sizeof(rbuf)));
if (n <= 0) break;
switch (PS(hash_func)) {
case PS_HASH_FUNC_MD5:
PHP_MD5Update(&md5_context, rbuf, n);
break;
case PS_HASH_FUNC_SHA1:
PHP_SHA1Update(&sha1_context, rbuf, n);
break;
#if defined(HAVE_HASH_EXT) && !defined(COMPILE_DL_HASH)
case PS_HASH_FUNC_OTHER:
PS(hash_ops)->hash_update(hash_context, rbuf, n);
break;
#endif
}
to_read -= n;
}
close(fd);
}
#endif
}
digest = emalloc(digest_len + 1);
switch (PS(hash_func)) {
case PS_HASH_FUNC_MD5:
PHP_MD5Final(digest, &md5_context);
break;
case PS_HASH_FUNC_SHA1:
PHP_SHA1Final(digest, &sha1_context);
break;
#if defined(HAVE_HASH_EXT) && !defined(COMPILE_DL_HASH)
case PS_HASH_FUNC_OTHER:
PS(hash_ops)->hash_final(digest, hash_context);
efree(hash_context);
break;
#endif
}
if (PS(hash_bits_per_character) < 4
|| PS(hash_bits_per_character) > 6) {
PS(hash_bits_per_character) = 4;
php_error_docref(NULL TSRMLS_CC, E_WARNING, "The ini setting hash_bits_per_character is out of range (should be 4, 5, or 6) - using 4 for now");
}
outid = emalloc((size_t)((digest_len + 2) * ((8.0f / PS(hash_bits_per_character)) + 0.5)));
j = (int) (bin_to_readable((char *)digest, digest_len, outid, (char)PS(hash_bits_per_character)) - outid);
efree(digest);
if (newlen) {
*newlen = j;
}
return outid;
}
正如您所看到的，实际上的id是一些东西的混合物的哈希值，例如当天的时间。因此，有可能会遇到冲突，但是这种可能性非常低。事实上，除非您有大量并发用户，否则不值得担心。
不过，如果您真的很担心，可以通过设置不同的哈希算法session.hash_function来增加熵。
至于监视活动会话，这个问题已经很好地涵盖在了这篇文章中 Is it possible to see active sessions using php?
如果您在单台机器上使用单个php实例，则它实际上具有内置的会话管理器，它会在分配id之前检查该id是否已存在。但是，如果您运行多个实例或多台机器，则无法知道其他机器分配的id。
