数组到字符串转换PHP ERROR [关闭]

In my database SQL, I have messages flowing between user a and b. I want to echo out all the messages they have had. But I am getting the error "array to string conversion" at the moment. I tried to use implode but that didn't work too well. How can I receive every row of messages flowing between both users?

$query = "SELECT message "
       . "FROM chat "
       . "WHERE (user_name='a' or user_name='b') "
       . "AND (user_to='b' or user_to='a')";
$sql = mysql_query($query);
$retrievedata = mysql_fetch_assoc($sql);
echo $retrievedata;

mysql_fetch_assoc fetches a single row, as an associative array (column name to value). In order to print all the messages, you'd have to go over the entire result set:

while ($retrievedata = mysql_fetch_assoc($sql)) {
    echo $retrievedata['message'];
}

Mandatory comment:
mysql* has been deprecated for several years now. Please use mysqli* or PDO instead.

Use var_dump($retrievedata) to print values of an array