PHP generation SessionID and picture check code ideas and implementation _php tutorial

To do a background login need to use the check code, the front-end user tracking needs to use the SessionID, of course, the default PHP opened the session after a SessionID, but I need their own, and can be stored in the database, then I tried a bit, constructs the following function.
/****** generates session ID ******/
The basic idea: is to take the current microsecond time to obtain, and then generate a random number, the random number and the current time after the addition of encryption, and finally intercept the required length
/*
Function name: create_sess_id ()
Function: Generates a random session ID
Parameters: $len: The length of the session string is required, default is 32 bits, not less than 16 bits
Return Value: Return session ID
function Heiyeluren
*/
function create_sess_id ($len =32)
{
Verify the length of the commit is legal
if (!is_numeric ($len) | | ($len >32) | | ($len <16)) {return;}
Gets the current time in microseconds
List ($u, $s) = explode (' ', Microtime ());
$time = (float) $u (float) $s;
Generate a random number
$rand _num = rand (100000, 999999);
$rand _num = rand ($rand _num, $time);
Mt_srand ($rand _num);
$rand _num = Mt_rand ();
Produce SessionID
$sess _id = MD5 (MD5 ($TIME). MD5 ($rand _num));
Intercept a SessionID that specifies the length required
$sess _id = substr ($sess _id, 0, $len);
return $sess _id;
}

/****** Generate check Code ******/
Idea: This idea is relatively simple, because consider uniqueness and randomness, our check code on the session ID to intercept a string, because our SessionID is fully considered unique.
/*
Function name: Create_check_code ()
Function: Generates a random check code
Parameter: $len: Requires the length of the check code, please do not longer than 16 bits, the default is 4 bits
Return value: Returns a check code of the specified length
function Heiyeluren
*/
function Create_check_code ($len =4)
{
if (!is_numeric ($len) | | ($len >6) | | ($len <1)) {return;}

$check _code = substr (create_sess_id (), $len);
Return Strtoupper ($check _code);
}

/****** generate a picture of the checksum code ******/
This is some of the more simple PHP graphics programming things up, I made the pictures and simple.
/*
Function name: Create_check_image ()
Function: Produces a picture of a checksum code
Parameters: $check _code: Check code string, generally by the Create_check_code () function to obtain

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