Find/replace mysql_ to mysqli_ makes code nonfunctional

Question

So as I'm sure anyone who's been a regular on SO has noticed, the mysql_ functions are going to be deprecated and it is suggested that one use mysqli_ or PDO instead. Thus, I decided to transition over to mysqli by doing a simple find/replace in my code, replacing every mysql_ with mysqli_. This seemed to work okay in Dreamweaver, and I got no syntax errors or anything. All the new mysqli_ functions were colored blue, meaning that they were recognized as valid functions.

However, when I saved everything and ran the code, I noticed that any code having to do with mysql had become nonfunctional. Undoing the replace solved the problem.

Is my server perhaps not supporting the mysqli functions? I know that it's running php 5 and mysql 5. Is there perhaps something else that you have to add to the code? What am I missing?

Answer 1

It's a good time to switch now, since PHP 7.0 removed the ext/mysql functions from core.

Consider the following legacy code

$res = mysql_query($sql);

The mysql_ functions were lazy, in that it would use whatever connection was open. But mysqli_ is not only not lazy, it's an object, not a connection resource (which is what mysql_connect() would return). Look at these lines here for connecting to the database

$mysqli = new mysqli('host', 'username', 'password', 'db');
$mysqli = mysqli_connect('host', 'username', 'password', 'db');

Both of them give you the same thing...

Returns an object which represents the connection to a MySQL Server.

Yes, that's right. Even the procedural connection function returns an object. The simplest way to use it is directly in is object form

$mysqli->query($sql);

The procedural functions are simply wrappers for this. But they are NOT lazy. Check out the difference

mysqli_query($mysqli, $sql);

The single largest gotcha in this (why a simple find and replace of mysql_ with mysqli_ fails) is that most of the mysqli_ functions require you to pass the connection object as the first argument. It's also important to note that results are objects as well (mysqli_result class), although their procedural counterparts only require the result set so you can simply find/replace those.

Just to be complete, there's one other gotcha that's less common (this is really old code): mysql_result. This function has no equivalent in mysqli_. You should switch to the full row return functions like mysqli_fetch_assoc($result)