Сообщение ошибки unknown system variable a

I used this code to catch errors
<?php
$query = "SELECT XXname FROM customer_table ";
$res = $mysqli->query($query);

if (!

$res) {
printf("Errormessage: %s\n", $mysqli->error);
}?>
The problem with this is that valid values for $res are: a mysqli_result object , true or false
This doesn't tell us that there has been an error with the sql used.
If you pass an update statement, false is a valid result if the update fails.

So, a better way is:
<?php
$query = "SELECT XXname FROM customer_table ";
$res = $mysqli->query($query);

if (!

$mysqli->error) {
printf("Errormessage: %s\n", $mysqli->error);
}?>

This would output something like:
Unexpected PHP error [mysqli::query() [<a href='function.query'>function.query</a>]: (42S22/1054): Unknown column 'XXname' in 'field list'] severity [E_WARNING] in [G:\database.php] line [249]

Very frustrating as I wanted to also catch the sql error and print out the stack trace.

A better way is:

<?php
mysqli_report(MYSQLI_REPORT_OFF); //Turn off irritating default messages$mysqli = new mysqli("localhost", "my_user", "my_password", "world");$query = "SELECT XXname FROM customer_table ";
$res = $mysqli->query($query);

if (

$mysqli->error) {
try {
throw new Exception("MySQL error $mysqli->error <br> Query:<br> $query", $msqli->errno);
} catch(Exception $e ) {
echo "Error No: ".$e->getCode(). " - ". $e->getMessage() . "<br >";
echo nl2br($e->getTraceAsString());
}
}//Do stuff with the result
?>
Prints out something like:
Error No: 1054
Unknown column 'XXname' in 'field list'
Query:
SELECT XXname FROM customer_table

#0 G:\\database.php(251): database->dbError('Unknown column ...', 1054, 'getQuery()', 'SELECT XXname F...')
#1 G:\data\WorkSites\1framework5\tests\dbtest.php(29): database->getString('SELECT XXname F...')
#2 c:\PHP\includes\simpletest\runner.php(58): testOfDB->testGetVal()
#3 c:\PHP\includes\simpletest\runner.php(96): SimpleInvoker->invoke('testGetVal')
#4 c:\PHP\includes\simpletest\runner.php(125): SimpleInvokerDecorator->invoke('testGetVal')
#5 c:\PHP\includes\simpletest\runner.php(183): SimpleErrorTrappingInvoker->invoke('testGetVal')
#6 c:\PHP\includes\simpletest\simple_test.php(90): SimpleRunner->run()
#7 c:\PHP\includes\simpletest\simple_test.php(498): SimpleTestCase->run(Object(HtmlReporter))
#8 c:\PHP\includes\simpletest\simple_test.php(500): GroupTest->run(Object(HtmlReporter))
#9 G:\all_tests.php(16): GroupTest->run(Object(HtmlReporter))

This will actually print out the error, a stack trace and the offending sql statement. Much more helpful when the sql statement is generated somewhere else in the code.

If you have the following situation

if (!$mysqli->query("SET a=1")) {
$mysqli->query("ROLLBACK;")
printf("Errormessage: %s\n", $mysqli->error);
}

you don't get an error-message, if the ROLLBACK-Query didn't failed, too. In order to get the right error-message you have to write:

if (!$mysqli->query("SET a=1")) {
printf("Errormessage: %s\n", $mysqli->error);
$mysqli->query("ROLLBACK;")
}

Initially, I used the below code to throw and subsequent catch mysqli exceptions

<?php
try {
$mysqli = new mysqli('localhost','root','pwd','db');
if ($mysqli->connect_errno)
throw new Exception($mysqli->connect_error);

} catch (

Exception $e) {
echo $e->getMessage();
}I realized the exception was being thrown before the actual throw statement and hence the catch block was not being called.My current code looks like
mysqli_report(MYSQLI_REPORT_ALL) ;
try {
$mysqli = new mysqli('localhost','root','pwd','db');
/* I don't need to throw the exception, it's being thrown automatically */} catch (Exception $e) {
echo $e->getMessage();
}This works fine and I'm able to trap all mysqli errors

So even if you escape everything in your SQL query using mysqli_real_escape_string(), make sure that if you plan to display the string returned by mysqli_error() you run that string through htmlspecialchars().

As far as I can tell the two escape functions don't escape the same characters, which is why you need both (the first for SQL and the second for HTML/JS).