Flow Control
- To work with if-elseif-else conditions in PHP.
- To work with switch/case statements in PHP.
- To work with loops in PHP.
Conditional Processing
Conditional processing allows programmers to output different code based on specific conditions. There are two conditional structures in PHP - if-elseif-else and switch/case.
If Conditions
Simple if statement
if (conditions) Do this;
In the above code, the Do this; statement will either run or not run depending on whether or not the conditions are true. This syntax can only be used when the condition affects a single line of code. For a block of code, use the following syntax.
if (conditions)
{
Do this;
Then do this;
And this too;
}The lines of code affected by the if condition are put in a code block, which is surrounded by curly brackets to indicate that all of the code either should or should not be executed, depending on the result of the if condition.
if-else statement
if (conditions)
{
Do this;
}
else
{
Do that;
}if-elseif-else statement
if (conditions)
{
Do this;
}
elseif (other conditions)
{
Do that;
}
else
{
Do this other thing;
}The two syntax blocks above show an if-else and an if-elseif-else statement, which can have any number of elseif blocks.
The following table shows PHP's comparison operators.
| Operator | Description |
|---|---|
| == | Equals |
| != | Doesn't equal |
| > | Is greater than |
| < | Is less than |
| >= | Is greater than or equal to |
| <= | Is less than or equal to |
| === | Identical (same value and same type) |
| !== | Not Identical |
The following example demonstrates an if-elseif-else statement.
Code Sample: FlowControl/Demos/If.php
<html>
<head>
<title>if-elseif-else</title>
</head>
<body>
<?php
$Age = 21;
if ($Age >= 21)
{
echo 'You can vote and drink!';
}
elseif ($Age >= 18)
{
echo 'You can vote, but can\'t drink.';
}
else
{
echo 'You cannot vote or drink.';
}
?>
</body>
</html>
The file is relatively simple. You can see the different results by changing the value of $Age.
Compound If Statements
More complex if statements often require that several conditions be checked. The table below shows and and or operators for checking multiple conditions and the not operator for negating a boolean value (i.e, turning true to false or vice versa).
| Operator | Name | Example |
|---|---|---|
&& and |
AND |
$a && $b |
|| or |
OR |
$a || $b |
! |
NOT |
!$b |
The following example shows these logical operators in practice.
Code Sample: FlowControl/Demos/If2.php
<html>
<head>
<title>if-elseif-else</title>
</head>
<body>
<?php
$Age = 21;
$Citizen = false;
if ($Age >= 21 && !$Citizen)
{
echo 'You can drink, but can\'t vote.';
}
elseif ($Age >= 21)
{
echo 'You can vote and drink!';
}
elseif ($Age >= 18 && $Citizen)
{
echo 'You can vote, but can\'t drink.';
}
else
{
echo 'You cannot vote or drink.';
}
?>
</body>
</html>
switch/case
A switch/case statement is similar to an if statement, except that it can only check for an equality comparison of a single expression. It cannot, for example, be used to check if one value is higher than another.
switch (expression)
{
case 'a' :
echo 'expression is a';
break;
case 'b' :
echo 'expression is b';
break;
case 'c' :
echo 'expression is c';
break;
default :
echo 'expression is unknown';
break;
}The break statement is important. Without it, after a single match is found, all following statements will execute.
The following example demonstrates a switch/case statement without break statements.
Code Sample: FlowControl/Demos/Switch.php
<html>
<head>
<title>switch/case</title>
</head>
<body>
<?php
$Quantity = 1;
switch ($Quantity)
{
case 1 :
echo 'Quantity is 1';
case 2 :
echo 'Quantity is 2';
default :
echo 'Quantity is not 1 or 2';
}
?>
</body>
</html>
The screenshot below shows the result.
Notice that, once a match is found, all remaining echo statements are output. The following example shows how this can be fixed by adding break statements.
Code Sample: FlowControl/Demos/SwitchWithBreak.php
<html>
<head>
<title>switch/case</title>
</head>
<body>
<?php
$Quantity = 1;
switch ($Quantity)
{
case 1 :
echo 'Quantity is 1';
break;
case 2 :
echo 'Quantity is 2';
break;
default :
echo 'Quantity is not 1 or 2';
}
?>
</body>
</html>
This time, only the first statement is output:
Loops
As the name implies, loops are used to loop (or iterate) over code blocks. The following section shows the syntax for different types of loops. Each loop will return "12345".
There are several types of loops in PHP.
- while
- do...while
- for
- foreach
while
while loops are used to execute a block of code repeatedly while one or more conditions is true.
$a=1;
while ($a < 6)
{
echo $a;
$a++;
}do...while
do...while loops are used to execute a block of code repeatedly until one or more conditions is found to be false. The difference between while loops and do...while loops is that the condition is checked after the code block is executed. This means that, in a do...while loop, the code block will always be executed at least once.
$a=1;
do
{
echo $a;
$a++;
}
while ($a < 6);for
A for loop takes three expressions separated by semi-colons and grouped in parentheses before the block to be iterated through.
- The first expression is executed once before the loop starts. It is usually used to initialize the conditions.
- The second expression is evaluated before each iteration through the loop. If it evaluates to false, the loop ends.
- The third expression is executed at the end of each iteration through the loop. It is usually used to make changes that can affect the second expression.
for ($a=1; $a < 6; $a++)
{
echo $a;
}break and continue
To break out of a loop, insert a break statement.
for ($a=1; $a < 6; $a++)
{
echo $a;
if ($a > 3)
{
break;
}
}To jump to the next iteration of a loop without executing the remaining statements in the block, insert a continue statement.
for ($a=1; $a < 6; $a++)
{
if ($a == 3)
{
continue;
}
echo $a;
}Flow Control Conclusion
In this lesson of the PHP tutorial, you have learned to write if-elseif-else conditions, switch/case statements, and several different types of loops.