Cannot display mysqli_error($con) ???

[Arthur Chan]

This PHP script fails to display an error message when I deliberately induced an error by providing a wrong B_ID from TableB (its an FK constraint):

<?php
$con = mysqli_connect("localhost", usr, pwd, instance);
mysqli_set_charset( $con, "utf8mb4" );
if (mysqli_connect_errno($con)) {
   die('Failed to connect to MySQL : ' .mysqli_connect_error());
} 
$aA= (int)$_POST['AA']; 
$aB= (int)$_POST['AB'];
$c3po= $_POST['C3PO'];

$qA = "SELECT A_ID from TableA WHERE A_ID='$aA'";
$rA = mysqli_query($con,$qA);
$numA = (int)mysqli_num_rows($rA);

$qB = "SELECT B_ID from TableB WHERE B_ID='$aB'";
$rB = mysqli_query($con,$qB);
$numB = mysqli_num_rows($rB);

if ($numA==1 && $numB==1){
    $newC= "call sp_intoTableC ('$aA', '$aB','$c3po')";
    $rNewC = mysqli_query($con,$newC);
    if ($rNewC) {
        printf("\"".$numA."\"". " and ". "\"".$numB."\"". " have been posted.");
    } else { echo mysqli_error($con); }  // INDUCE ERROR: THIS DOES NOT GET DISPLAYED !?!
}
mysqli_close($con);
?>

Thanks.