Database connect: Why a variable?

rodnice

$connect = mysql_connect("localhost","peter","abc123")
$db=mysql_select_db($dbname,$connect) or die("Unable to connect to db");
$result = mysql_query($sql, $conn)or die("Unable to query dbl");

This is my first ever post here. I'm on chapter 6 of a 17 page
PHP4 book and I'm not understanding some minor things here
and there.

I did a ColdFusion site as a project several years ago and I still
think that was super easy to learn from the book I had, however,
no one loves ColdFusion so now I have to go along to get along.

Question:
Why do we put the connection into a variable? I'm thinking you
tell the system to connect and it stays that way till you end it or
it times out.

I also don't know why we put the above second code line and
above thrid line into a variable.

I might never get it and just have to live with it, but if I had to
teach this to someone one day, I would rather like to have an
answer better than "Because that's the way it is."

Thanks in advance guys

Weedpacket

You might want to open a connection to more than one database, and if you do you'd want to be able to tell them apart. "Send this query to the database." "Which one?". "Close the connection." "Err....?"

If you only have one database connection open, then the information is redundant. Have a look at what the manual says about [man]mysql_select_db[/man] and [man]mysql_query[/man]: in both cases the connection argument is optional. (Incidentally, since PHP 5 there has been a new database model, [man]PDO[/man], which is a bit more powerful).

bretticus

rodnice;10893643 wrote:
$connect = mysql_connect("localhost","peter","abc123")
$db=mysql_select_db($dbname,$connect) or die("Unable to connect to db");
$result = mysql_query($sql, $conn)or die("Unable to query dbl");

Question:
Why do we put the connection into a variable?

Because you might want to use multiple connections in a script. Because, a connection variable is portable. Because this is pretty much how every other language that supports RDMS in existence does it. 😃

rodnice;10893643 wrote:
I also don't know why we put the above second code line and
above third line into a variable.

You don't have to set mysql_select_db to a variable in your example. Since, you are dying on a failed db connect, that $db will never need be used.

Finally, assigning a result to mysql_query gives you the convenience of getting number of rows to decide if you're going to loop through any rows ( mysql_num_rows() ) and let's you choose how you want to loop through the results (numerous functions that use a "mysql resource".)

Modularity in a language is an enabling virtue. If you want an all-in-one function for querying data records, there are many database abstraction layers written for PHP (or roll your own.)

Peace!

Oh...(looks like I typed very slow once more...already answered by weedpacket) one more thing...I'm not suggesting trading in your PHP4 book, but when you get to OOP, you may wanna skim it lightly and then read the manual about PHP5 OOP. PHP4 is pretty much obsolete.

rodnice

Ok, I guess I just gotta get used to it. Let me let your answers soak
in a bit.

PHP5? Yeah, ok I'll have to hop on that, but my only fear is that someone
will require me to know PHP4 for a job or something and saying "Well PHP4
is pretty much obsolete" may not save my hide (kinda like how they told me
that of COBOL 10 years ago but people still require those programmers).

I guess I'm used to the ease of ColdFusion.

bretticus

Well, not having used coldfusion, but having run across the syntax somewhat, cold fusion is more of a templating engine. PHP, on the other hand, is a little more like a typical programming language (not just for Web pages.) I must admit coldfusion is convenient. To make templating easier with php, developers have used PHP libraries such as Smarty to offer what you are more used to with CF. Personally, I'm a big fan of MVC Frameworks for PHP. In particular, I like CodeIgniter. However, I suggest you get the basics down (even though, at the "page" level, the MVC model is more similar to CF development.)

PHP4 and PHP5 are essentially the same. There are just more alternatives with PHP5. There are also subtle differences in how PHP5 implements OOP (object oriented programming.) Since PHP5 is (for the most part) backwards compatible with PHP4, learning PHP5 isn't going to cost you cany jobs IMHO 😉

Peace.

rodnice

Ok, so does the following 2 pieces of code accomplish the same thing?

First:
mysql_connect("localhost","peter","abc123")

Second:
$connect = mysql_connect("localhost","peter","abc123")

NogDog

Both statements execute the mysql_connect() function. The only difference is that in the second version the value returned by the function is saved in a variable making it available for use later should you need to, while in the the first it is simply discarded. Neither version is necessarily better or worse, though having the return value saved in a variable from the get-go means that if at some point in the evolution of your code you need to access that value, you already have it (not that it would be a big deal to add it later if needed).

However, as your coding skills and style improve and your code becomes more modular, you may find that you always want to save the result in a variable as your code for basic functionality like connecting to a DB will likely get re-used in many applications/functions, so going with the increased capability of saving the result in a variable might enhance your ability to modularize things.

Then again, you may find that last sentence needlessly complex and as difficult to read as uncommented procedural PHP code written by a web designer, so who knows? 😉

laserlight

rodnice wrote:
Ok, so does the following 2 pieces of code accomplish the same thing?

In addition to what NogDog stated, you might have noticed that Weedpacket mentioned the PDO extension (which I also recommend over the MySQL extension). If you use the PDO extension, you pretty much have to explicitly save the connection object.

rodnice

Basically, the result of any equation or other derived value we would generally save to a variable. Doing this is good coding practice.

Do I have it right now?

NogDog

"Generally" is probably the operative word here. It's certainly not always needed. For instance, it's probably normally not necessary to save the return value of a call to [man]print/man. But when in doubt, you might as well save a return value if you think you might possibly need it at some point. Most servers are running with enough memory, disk space, and CPU power that the additional resources needed to save something like a connection resource are trivial.