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Doc Bug #69884 mysqli->connect_error seems to be shared across objects
Submitted: 2015-06-19 14:19 UTC Modified: 2018-08-14 02:54 UTC
Avg. Score:4.7 ± 0.5
Reproduced:3 of 3 (100.0%)
Same Version:0 (0.0%)
Same OS:1 (33.3%)
From: phpbugs at olemartin dot org Assigned:
Status: Verified Package: MySQLi related
PHP Version: 5.5.26 OS: Ubuntu
Private report: No CVE-ID: None
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 [2015-06-19 14:19 UTC] phpbugs at olemartin dot org
It seems that the \mysqli->connect_error and \mysqli->connect_errno values (when using mysqli in the object-oriented mode) shares state between all connections. This is not documented (it should at least have a warning in the documentation).

When running the example code (make sure you point the first and last connection to a valid MySQL server) i found that all connections reported no errors, although the second one failed. Sometimes (although I'm having a hard time reliably reproducing this) i'm pretty sure i've also seen all three connections report errors (although the third and last connection succeeded). Maybe this value is not overwritten when a successful connection is done, in all code paths?

Either way, i think this should be either documented or fixed - as one would not expect this information to be shared between instances when the syntax doesn't indicate that. If this is a static property, i would expect the documentation said to use \mysqli::$connect_error, but that doesn't even work.   

Test script:
$connections = array(
    'Valid connection' =>   new mysqli('', 'root', '', 'test', 3306),
    'Invalid connection' => @new mysqli('', 'root', '', 'test', 3307),
    'Valid connection 2' => new mysqli('', 'root', '', 'test', 3306),

foreach($connections as $desc => $conn) {
    echo "$desc:\n".
        "connect_error = ".$conn->connect_error."\n".
        "connect_errno = ".$conn->connect_errno."\n".
        "ping =          ".@$conn->ping()."\n\n".

Expected result:
Valid connection:
connect_error =
connect_errno = 0
ping =          1

Invalid connection:
connect_error = Connection refused
connect_errno = 2002
ping =          

Valid connection 2:
connect_error = 
connect_errno = 0
ping =          1

Actual result:
Valid connection:
connect_error = 
connect_errno = 0
ping =          1

Invalid connection:
connect_error = 
connect_errno = 0
ping =             

Valid connection 2:
connect_error = 
connect_errno = 0
ping =          1


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 [2018-08-13 21:14 UTC] brak at gameservers dot com
I'm also seeing this same behavior on PHP 7.1.19 running on CentOS 7
 [2018-08-14 02:54 UTC]
-Status: Open +Status: Verified -Type: Bug +Type: Documentation Problem
 [2018-08-14 02:54 UTC]
The values are indeed "shared", meaning the values are not associated with a specific instance but set globally.
 [2020-10-19 09:58 UTC] call dot user dot func at gmail dot com
Hi, this behaviour is the same in php8+. A warning in the docs would've been really helpful, as it's a pain to debug.

A very brief testcase:
$db1 = new MySQLi("localhost", "root", "", "mysql", 3306);
var_dump("1: ".$db1->connect_error); // not error, db exist

$db2 = new MySQLi("localhost", "root", "", "some_non_existing_db", 3306);
var_dump("2: ".$db2->connect_error); // error, db not exist

// db2 shared error to db1 which is wrong, db1 is still valid
var_dump("3: ".$db1->connect_error); // not error, db exist
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