In any given computer language (PHP is no exception) there has to be a way to allow the running code to decide between doing 2 different things.
If there wasn't then software would not be able to adapt based on operating conditions, or it wouldn't be able to decide between two different users.
In most languages the most basic of these is the "if" statement, which as its name implies does something if a given condition is true.
Let's look at a quick example:

  $name = "peter";
  if($name == "peter")
    print "Hello peter how are you today?\n";
    print "Sorry I don't know you\n";

As you can see it's very simple. First we have a string variable that holds a name. Then we have the "if" statement which says 'if the variable called name holds a value that is equal to peter then perform the program lines in the first set of curly brackets, otherwise perform the commands in the second set of brackets'
The first question you might ask is why do we have a double == sign, good question. This is because in PHP everything in a decision has to result in either true or false, and since we use = to give a value to a variable, then = will always be true (trust me, computers work that way ;-) ) and thus we would not be able to get a false. There are some cases where you have to use === (3 = in succession) but for most of the time == will do just fine.
Other Checks?
We don't just have to check to see if two values are equal, we can also check to see if they are not equal, or greater than and less than, the following list shows the different operators 'if' understands:
You can also group one or more decisions together by using normal brackets, separated by any of the Boolean operators as shown below:
These are typically used in the following way:

  $fname = "peter";
  $lname = "shaw";
  if( ($fname == "peter") && ($lname == "shaw") )
    print "greetings Shawty, you are allowed in…";
    print "be-gone stranger, you are banished from here…";

This is now saying that fname must be equal to "peter" AND lname must be equal to "shaw" before I allow the top code to run, if not then the bottom code will run.
The full range of operators can be found in the php manual here and as always I strongly encourage you to read and experiment with different combinations.
1,2,3 hey look I'm counting
The second statement in our journey is the "for" loop, the whole purpose of which is to repeatedly do something until a given condition is true. In some ways it's kind of like a repeating "if" statement as shown in the example below:

    print $counter . "\n";

At first glance this looks very complicated, but if we take it apart bit by bit, you'll see it suddenly makes a lot of sense:
So, as you can see the loop contains 3 parts, an initialization part, a when to stop check, and an update part, all together providing you fine control over how many times something happens.
What if I don't know in advance how many there is going to be?
That's a perfectly valid question. You can't always know in advance how many of a given count there are going to be, or even where that count will start.
For this reason we have the while and do-while loop operators, and just as they sound by their names, they keep performing a given task until something is true, eg.

$stop = 0;
while($stop != 1)
{   Print "Still running\n";
  $stop = someFunctionThatReturns1or0();

Again, the layout is very simple. In the brackets we have a decision in exactly the same way as in an 'if' and yes you can group using && and || too, and then we have a block of code inside our curly brackets, that keeps going until our fictitious function returns a 1.
While can check anything you like in exactly the same fashion, EG:

  $line = getNextLine();
  while($line != "peter")
    $line = getNextLine();

The while loop will keep calling "getNextLine" until the contents of $line are equal to peter. NOTE: the "getNextLine" and "someFunctionThatReturns1or0" calls are for example only, so don't just cut and paste them, they won't work because they don't really exist.
Do-while is a reverse version of while, and anything in the curly braces will be executed at least once before the check to stop is encountered. Using our example above:

    $line = getNextLine();
  } while($line != "peter");

As you can see, the first thing this does is prevent us from having to load our variable the first time, because we know that the code will run at least one time, which while loop you use will depend on exactly how your program logic is to function.
We covered the most commonly used control statements here, but there are other controls available in the language, such as 'switch' and dare I say it (Shudders at the thought) 'goto', which in mine and probably most professional programmers minds should never have been invented in the first place. There are also a couple of variations on the standard if statement that I've not covered. As always refer to the PHP manual's section on control statements at .control-structures.php and experiment with the others. I would also strongly recommend reading the user submissions in this section, even if you haven't in any of the others, some of the tips and shortcuts in this section show some extremely cool tricks that make PHP come alive.
Until next time
Don't get stuck in a loop.
The ABC's of PHP
Introduction to PHP
What do I need to make it work?
Basic Script Building in PHP
How Variable Am I?
Strings & Text
Math & Number Handling in PHP
Introduction to Arrays and Hashes in PHP
Loops and Decisions in PHP
Advanced String Processing - How Regular Are Your Expressions