We are affected by this bug and it's very important for us because our framework relies on valid return values for "affected_rows".

We can reproduce this issue on Debian Jessie with package php5 (5.6.x) and xdebug installed.

This is a recipe to make a developer crazy. You hit a bug in your code, add breakpoints to try debugging it, and get crazy results. I can't believe this persists. I am experiencing this under PHP 5.5.12.

This was reported by figiemmi to NetBeans Forums on 2012-11-24, but he did not specify which PHP version he was using. This is visibly an old bug, but it's hard to believe it's always been that way.

This was also reported against NetBeans: https://netbeans.org/bugzilla/show_bug.cgi?id=244714

Thank you very much for reporting, your test case and your analysis, Derick.

I should have specified this affected me while using XDebug on Windows (8.1). This happened when I migrated an application from mysql to mysqli, which is its closest equivalent. And the reason why I migrated from mysql is that I upgraded to 5.5... which deprecates the mysql extension! Unfortunately, there will probably many others affected.

And unfortunately, the procedural style mysqli_affected_rows() is also affected, so the best workarounds I see is to go back to mysql, go to PDO instead, or wrap the critical part of our high-level query() function in big "DO NOT STEP THROUGH THIS CODE" comments. We wouldn't have the last choice if we didn't use such a wrapper, so we're in fact among the luckiest...

I have this problem regardless of whether I step through the code in my Eclipse/Xdebug/MAMP environment.  It seems if I try to open/close mysqli connections when used, the problem occurs.  If I don't close the connections, I get no errors about accessing properties, but I do then get the expected "too many connections."  Any chance this will get fixed soon?  It's really hard to debug when your error logs fill with hundreds of lines of useless info.

		if(!$this->db->isConnected()){
			$this->db->selectDB();
			$this->dbc = $this->db->dbc;
		}
		$this->resource = $this->dbc->query($this->text);
		if($this->resource===false){
			$this->error=$this->dbc->connect_error;
			if(isset($this->log)){
				$err=$this->log->writeLog($this->text."\n".$this->error."\n".$this->implode_r("\n",debug_backtrace()),'log',$tag);
			}else{
				echo $this->text."\n".$this->error."\n".$this->implode_r("\n",debug_backtrace()),'log -- '.$tag;
				die;
			}
		}elseif(strpos($this->text,'insert into')!==false){
			$this->result=$this->dbc->insert_id;
		}elseif(strpos($this->text,'delete')!==false or strpos($this->text,'update')!==false){
			$this->result=$this->resource;
		}else{
			$this->fetch_all();					
		}
		$this->dbc->close();

Happens for me as well on PHP 5.4, PHP 5.5, PHP 5.6. Maybe even on PHP 7.0.

Same here.
In my phpunit tests I connect to mysql using mysqli_connect on setUp().
All is ok when the first test runs mysqli_real_escape_string.
But when it is not the first test running then I get the below:

mysqli_real_escape_string(): Couldn't fetch mysqli

I am running PHP7.0 on PHPUnit 5.6.1

Hello everybody, 

the source of all this problem in in $dbc->stat triggering a COM_STATISTICS command to the server which will update affected_rows to -1. This is expected since no rows were influenced by COM_STATISTICS so make no sense to keep the old value 1 (or whatever other value we have stored). 

Being said that, we're thinking about a removal of the $dbc->stat function which will remove the trigger of the command to the server, instead $dbc->stat will read the plain value from the object, this shall fix this problem.

I can confirm that this happens in php 7.0.22 as well.

I can confirm that this happens in php 7.2.11 as well.

I can confirm that this happens in php 7.2.11 as well.
(mysqli->info: "Rows matched: 1  Changed: 1  Warnings: 0")

This is still an issue in PHP 7.2.2

I have spent days trying to figure out why the code I inherited works on the server but not on my local machine. "mysqli_affected_rows" was the culprit as this is what the previous devs used quite often. 

Is there anyone working on the fix? Is this issue being addressed at all? I would love to contribute but I do not have the skills, unfortunately.

This is going to be in 7.4.0alpha2 and later, and the fix is to remove the stat property. The stat() method to obtain the same information is still available.

Direct removal of a property, part of API, without deprecation. What's going on here?!?

There have been 5 years since the report of this bug, and nothing happening. This bug involves the MySQLi extension *destroying* data when reading this undocumented property (https://www.php.net/manual/en/class.mysqli.php) or doing a var_dump() on a MySQLi object. If you can fix this data trashing bug in another way, then feel free to attempt this.

It's worth noting that the mysqli::$stat property was undocumented -- per documentation only mysqli::stat() exists, and of course that remains available.