Looping Statements With PHP5.3

by Marc Plotz

August 18, 2009

Introduction

Looping statements are the backbone of PHP. I use them on a daily basis and they make things really simple. But I always think back to the days when everything except the WHILE loop looked alien to me and I wonder if anyone else feels the same? Do you wonder what those annoying looking $i variables are doing in that FOR loop? Or what's that funny arrow in the FOREACH loop? It all really is much more simple than you think. Let's have a look.

WHILE

This is probably the most simple of all loops, and pretty much does what its name implies. The loop will iterate WHILE a statement is true. In other words, while a certain condition is met, it will continue running through a loop. What actually happens here is that the condition is checked and if it comes back true the rest of the code in the statement is run. Lets have a look at an example:

$query = mysql_query("SELECT * FROM TABLE");


if($query)

{

    while($result = mysql_fetch_array($query))

    {

        echo $result['fieldname'];

        echo '<br />';

    }

}

What the above will do is loop through the table for as long as there is a result, or in other words another row in the database. This is a very simple looping statement, but in simplicity lies danger: this little exercise, when running through large database sets can be extremely heavy on resources. Most of those resources are wasted in the mysql_fetch_array that is reiterated on every loop of the statement.

FOREACH

FOREACH is similar to WHILE, except for a few things. Have a look at the example below:

foreach($array as $key => $value)

{

echo $key.' '.$value;

echo '<br />';

}

What we see above is a statement that basically says: For each element of the $array associative array I want to refer to the key as $key and the value as $value. The operator => refers to the relationship between the value and the key. I like to think of it as the key pointing to its value. Another, more simple way of running this statement actually ignores the key and simply works with the values as an array. We explore this below:

foreach($products as $product) 
{ 
    echo $product['name']; 
    echo '<br />'; 
}

The real simplicity in the for loop is that it is lighter on resources, it is not checking whether a condition is true or false, it is not deciding what to do. Server resources are extremely important, specially when you are running enterprise class applications. Of course, something like that will naturally have its own database or display classes to deal with these types of things. However, somebody had to write those classes, right? And at the end of the day it all comes back to what we are talking about here: whether you are working on Joomla, Zend Framework, Magenta, or a custom-built framework - when you strip the code down to the bone what remains is just normal, simple, boring, every-day statements, switches and, yes, looping statements. Getting used to these things now will make you a better coder by far, fast! Anyway, back to the code!

FOR

This was the one that I took the longest to understand. BUT, now that I understand it, this is the looping statement I use the most. Mostly due to its flexibility. An example below explains:

$count = count($array) 
for($i=0; $i < $count; $i++;) 
{ 
    echo $array[$i]['name']; 
    echo '<br />'; 
} 

What we are basically saying in the first row is that we begin at the first key in the array ($i=0) and while $i has a value less than the amount of rows in the array ($i >= $count;) keep iterating through the loop ($i++). So, for every iteration of the statement where $i is lower than the total amount of rows in the array we shift the value of $i up one and continue looping. Obviously, when there are no more rows in the array we need to stop.

There are a few interesting things to note with the for loop. The first is that I executed the count of the array outside of the statement. This is important. Wherever possible, do not put queries, counts, statements or even more loops inside a looping statement. Imagine getting hundreds of thousands of entries from a database. Now imagine that for every entry you count how many entries there are. I'm guessing you are going to get old waiting for that query to execute. Executing what only needs to be executed once outside the loop is good practice. Once a variable has a value it doesn't need to recalculate it for every loop, the variable value is set for that entire script run and is just referenced as need be.

Something else to look at is the part that says ($i < $count;). What this is looking for is if the $i variable or counter has a value less than the count of entries in the array. But, you may ask, shouldn't it be executing while the counter has a value less than or equal to the count of array entries? Well. That is exactly what is happening, it just does not look that way at first glance. What you need to remember is that the array counter starts at 0, not one, in this instance. Now imagine there are 52 entries in the array. Thus, $i reaching 51 will actually be the 52nd entry, since the count started at 0 and not 1. Therefore only executing while the counter is less than 52 means that the counter will execute 52 times but have a value of 51 on its final iteration.

Conclusion

Looping statements are the backbone of PHP code. We use it daily, looping through lists, arrays, database entries and such. Understanding them is more important than just about anything else in PHP. It means nothing that you can get the data out of the database if you can do nothing with it at the end of the day. Therefore I have briefly touched on the simple workings of the three most popular looping statements, showing you simple examples of how they work and why they look like they do.

As always, happy LOOPING!