CREATE TABLE `prob` (
`id` INT UNSIGNED NOT NULL AUTO_INCREMENT,
`key` VARBINARY(4) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE = InnoDB CHARSET=utf8 COLLATE utf8_general_ci;
INSERT INTO `prob` ( `key` ) VALUES ( 0x1234 );

$key = pack( "H*", '01020304' );
if( $stmt = $con->prepare("INSERT INTO prob VALUES ( DEFAULT, ? )") ) {
    $stmt->bind_param( "s", $key );
    $stmt->execute( );
    $stmt->close( );
} else { $err = $con->error; echo "Err $err<br>"; }

if( $stmt = $con->prepare("SELECT id FROM prob WHERE key='?'") ) {
    if( !$stmt->bind_param( "s", $key ) ) {echo 'Err bind_param.<br>'; die( );}
    if( !$stmt->execute( ) ) {echo 'Err execute.<br>'; die( );}
    if( !$stmt->bind_result( $id ) ) {echo 'Err bind_result.<br>'; die( );}
    $rv = $stmt->fetch( );
    $stmt->close( );
    echo 'Result: '.print_r( $rv ).'<br>';
} else { $err = $con->error; echo "Err $err<br>"; }

if($result = $con->query( "SELECT id FROM prob WHERE key=$key" )) {
    if( $count = $result->num_rows == 1 ) {
        $row = $result->fetch_row( );
        $rv = $row[0];
        $result->close( );
    }
} else { $err = $con->error; echo "Err $err<br>"; }