PHP-Control Statements

Control statements are fundamental to programming languages. Control statements permit you to take some action if something is the case, and different action (or no action) otherwise. There are three common control statements in PHP:
    if . . . else: As the words suggest, this statement tests if something is true. If it is, then it does one thing, else it does another thing. The else part can be omitted. For example, 
    $IP_address = getenv("REMOTE_ADDR");
if ( $IP_address == "121.114.221.111" ) {
  echo "Hello, George.  Welcome to my web site.";
}
else {
  echo "Howdy, stranger.";
}

$temperature = 71;
if ( $temperature >= 50 ) {
  echo "Nice day!";
}
  1. while . . . do: This statement keeps repeating specified statements again and again, so long as a particular condition is true. The statements that are repeated should somehow affect the condition. Otherwise, the statement may never become false and the program will go into an infinite loop. Here is an example of a while loop:
    2. $a = 2;
while ( $a < 100 ) {
  echo " $a";
  $a *= $a;
}

$b = 1;
while ( $b < 10 ) {
  $a++;
} // OOPS!  This is an infinite loop!
  2. for . . . do: This is basically shorthand for a while loop where the condition is whether a particular variable has yet to reach a certain value. Each time the loop is executed the variable is generally incremented or decremented. This kind of control statement is useful when you want to count through a series of numbers or objects, or repeat a set of statements of particular number of times. For example,
    3. $sum = 0;
for ( $a = 1; $a <= 10; $a++ ) {
  $sum += $a;
}

echo "Sum of numbers from 0 to 10 equals $sum.";

//
// The for loop is just shorthand for the following while loop:
//
$a = 1;
while ( $a <= 10 ) {
  $sum += $a;
  $a++;
}
As you can see from the examples, the condition is tested using commonsense mathematical expressions. Indeed, here are the PHP comparison operators:
> (greater than)
< (less than)
<= (less than or equal to)
>= (greater than or equal to)
== (equal to; note that two equal signs are used to distinguish it from an assignment)
!= (not equal to)
There also are Boolean operators that allow you to combine conditional expressions:
&& (and)
|| (or)
! (not)
For instance, ( $a > 5 ) && ( $a < 20 ) tests whether the value of $a is greater than 5 and less than 20. The expression ( $name != "John" ) || ( $age >= 18 ) tests whether $name is not equal to "John" or $age is greater than or equal to 18.

Finally, !( ( $sky == "cloudy" ) || ( $temperature < 50 ) ) tests whether it is not the case that $sky equals "cloudy" or $temperature is less than 50. For instance you might write the following:  

$sky = "sunny";
$temperature = 65;

if ( !( ( sky == "cloudy" ) || ( $temperature < 50 ) ) ) {
  echo "Nice day.";
}
Else {
  echo "Lousy day.";
}
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