PHP mysql错误，无法INSERT到数据库

I have these lines of PHP code:

$sql2 = "INSERT INTO TableName ... " //an example, line is very long
error_log($sql2);
$result2 = $this->conn->query($sql2); //line 81

I get this error in PHP error log:

PHP Warning:  mysqli::query(): Couldn't fetch mysqli in /volume1/web/DB_Functions.php on line 81

My first idea was that my $sql2 string is not valid, but when I copied it from PHP error log and pasted it to PhpMyAdmin as SQL it works pretty well, what could be wrong?

I think it is because when you close the database connection the first time, you forget to do:

unset($this->conn);

And then when you try connecting to the database again, it craps out because it is still set to the closed connection. Originally referenced from: Warning: mysqli_query(): Couldn't fetch mysqli

Hope it solve your problem!