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up vote 4 down vote favorite

I have read about the function on php.net and that has still not answered my question. I know a beginners amount of C and I've just started using php. Normally in C if you were to do a while loop there needs to be some condition to advance the loop to a point where it will no longer be valid like so: 

while (x >= 10)  
{ 
    printf("..."; 
    printf("x \n"; 
    x++; 
}

However in my php script that I'm using for a pm message system I have a while loop like this: 

while($row2 = mysql_fetch_array($query))

followed by: 

{ 
  echo "<table border=1>";
  echo "<tr><td>";
  echo "Message #: ";
  echo $row['id'];
  echo "</td></tr>";
  echo "<tr><td>";
  echo "To: ";
  echo $row['to'];
  echo "</td></tr>";
  echo "<tr><td>";
  echo "From: ";
  echo $row['from'];
  echo " ";
  echo "</td></tr>";
  echo "<tr><td>";
  echo "Message: ";
  echo $row['message'];
  echo "</td></tr>";
  echo "</br>";
?>
<form action="<?php echo $_SERVER['PHP_SELF']?>" method="post">
<table border="0">
<tr><td colspan=2></td></tr>
<tr><td></td><td>
<input type="hidden" name="id" maxlength="32" value = "<?php echo $row['id']; ?>">
</td></tr>
<tr><td colspan="2" align="right">
<input type="submit" name="delete" value="Delete PM # <?php echo $row['id']; ?>">
</td>
<td colspan="2" align="right">
<input type="submit" name="reply" value="Reply to <?php echo $row['from']; ?>">
</td></tr>
</table>
<?php } ?>

How exactly does this work, from a C background it would seem almost like this would stay in the same spot, printing out the same "array fetch" 'row' from the same "$query" every time we go through the loop....

Is there a way I can write this to give me a better logical understanding of whats going on? like saying: 

$i=0;
while ($row = ($i+mysql_fetch_array($query)) {
...
...
$i++;}

I know that probably wont work but how does this function increment itself? And is there a way to write it where it would actually have some sort of incrementation visible in the code?

Thank you

share|improve this question
    
I would read up on the PHP documentation for this to get a good understanding. The function is basically working on a resource handle to your query results. The function will return FALSE if there are no more results to return and thus the while loop evaluates to FALSE. –  Mark May 8 '12 at 17:18
    
Please stop writing new code with the ancient mysql_* functions. They are no longer maintained and community has begun the deprecation process. Instead you should learn about prepared statements and use either PDO or MySQLi. If you care to learn, here is a quite good PDO-related tutorial. –  Second Rikudo May 8 '12 at 17:23

3 Answers 3

up vote 8 down vote

Every time you call mysql_fetch_array it pulls the next row from your query. That while loop keeps returning true while the mysql_fetch_array still has something left to assign to the variable $row2. Once it's out of rows, it has nothing left to give the variable, and false is returned.

ETA: Regarding the last bit you mentioned, you can have a variable increment in each iteration of the loop like in your example, but it's not entirely necessary. You can also just see how many rows have been returned by doing something like $var = mysql_num_rows($data) before your while loop.

share|improve this answer
    
Ok so say instead of just printing out each message stored in the database one by one (which is being done in my code above), I could instead save each iteration of the mysql_fetch_array function within an object in order to go through and arrange the messages on the page in alphabetical order, in order, to group messages from the same people together? If so would there be an easier way to do this without having to save the $row variable as an object, or would that actually make my code cleaner and easier to understand what's going on? And would objects like that slow down my page? Thank you –  Adam Johnston May 8 '12 at 17:44
    
It's definitely possible to store them; your code simply echos bits and pieces of each line, but you could just as easily assign the entire row (or just parts of it) to objects and/or arrays. Personally, when grouping things, I've always done it on the fly. I order the items in the SQL query, set an empty variable, (e.g. $group), then do a check before doing my output: if ($row2['name']!=$group) { $group = $row2['name']; // More code here } –  David Millar May 8 '12 at 18:02
up vote 1 down vote

You can think of mysql_fetch_array() as similar to a file read function that will return return a new row each time and return EOF when it hits the end. Instead of EOF though, mysql_fetch_array() returns FALSE(0).

In a Boolean expression, PHP evaluates any non-0 value as TRUE, so the simple syntax below loops through, assigning the row to the $row variable each time until we reach the end of the dataset:

while($row = mysql_fetch_array($query)) {
  // Process row
  ...
}

PHP IDEs will actually complain about an assignment in a condition, and this is how you should write the same code to avoid the notification:

// Get first row, or false if there were no rows
$row = mysql_fetch_array($query);
while ($row) {
  // Process row
  ...
  // Get next row
  $row = mysql_fetch_array($query);
}

Perhaps this structure looks more familiar.

share|improve this answer
up vote 0 down vote

mysql_fetch_array() is a function that you must call if you want to get a row from the resulted executed query, so you can describe it "mysql_fetch_array — Fetch a result row as an associative array, a numeric array, or both"

The Scenario is this : It returns an array that corresponds to the fetched row and moves the internal data pointer ahead. More Clarification : 

$result = mysql_query("SELECT id, name FROM mytable");
$row = mysql_fetch_array($result, MYSQL_NUM);

1) What is the type of $result ? 2) What type mysql_fetch_array uses to work ?

Let me answer this, 1) $result is a resulted variable from mysql_query(), and this function returns results typed as mysql result and this type can only be created by calling 7 functions :

mysql_db_query(), mysql_list_dbs(), mysql_list_fields(), mysql_list_processes(), mysql_list_tables(), mysql_query(), mysql_unbuffered_query()

and can used by several functions like mysql_fetch_array() and mysql_db_name() and others

2)The description of the function is : array mysql_fetch_array ( resource $result [, int $result_type = MYSQL_BOTH ] ) see mysql_fetch_array Doc

share|improve this answer

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