Flow Control
This chapter is from the book
This chapter is from the book
This chapter is from the book
In this lesson you will learn about the conditional and looping constructs that allow you to control the flow of a PHP script.
In this chapter we'll look at two types of flow control: conditional statements, which tell your script to execute a section of code only if certain criteria are met, and loops, which indicate a block of code that is to be repeated a number of times.
Conditional Statements
A conditional statement in PHP begins with the keyword if, followed by a condition in parentheses. The following example checks whether the value of the variable $number is less than 10, and the echo statement displays its message only if this is the case:
$number = 5;
if ($number < 10) {
echo "$number is less than ten";
}
The condition $number < 10 is satisfied if the value on the left of the < symbol is smaller than the value on the right. If this condition holds true, then the code in the following set of braces will be executed; otherwise, the script jumps to the next statement after the closing brace.
As it stands, the previous example will be TRUE because 5 is less than 10, so the statement in braces is executed, and the corresponding output is displayed. Now, if you change the initial value of $number to 10 or higher and rerun the script, the condition fails, and no output is produced.
Braces are used in PHP to group blocks of code together. In a conditional statement, they surround the section of code that is to be executed if the preceding condition is true.
Braces are not required after an if statement. If they are omitted, the following single statement is executed if the condition is true. Any subsequent statements are executed, regardless of the status of the conditional.
Conditional Operators
PHP allows you to perform a number of different comparisons, to check for the equality or relative size of two values. PHP’s conditional operators are shown in Table 3.1.
Table 3.1 Conditional Operators in PHP
|
Operator |
Description |
|
== |
Is equal to |
|
=== |
Is identical to (is equal and is the same data type) |
|
!= |
Is not equal to |
|
!== |
Is not identical to |
|
< |
Is less than |
|
<= |
Is less than or equal to |
|
> |
Is greater than |
|
>= |
Is greater than or equal to |
Logical Operators
You can combine multiple expressions to check two or more criteria in a single conditional statement. For example, the following statement checks whether the value of $number is between 5 and 10:
$number = 8;
if ($number >= 5 and $number <= 10) {
echo "$number is between five and ten";
}
The keyword and is a logical operator, which signifies that the overall condition will be true only if the expressions on either side are true. That is, $number has to be both greater than or equal to 5 and less than or equal to 10.
Table 3.2 shows the logical operators that can be used in PHP.
Table 3.2 Logical Operators in PHP
|
Operator |
Name |
Description |
|
! a |
NOT |
True if a is not true |
|
a && b |
AND |
True if both a and b are true |
|
a || b |
OR |
True if either a or b is true |
|
a and b |
AND |
True if both a and b are true |
|
a xor b |
XOR |
True if a or b is true, but not both |
|
a or b |
OR |
True if either a or b is true |
You may have noticed that there are two different ways of performing a logical AND or OR in PHP. The difference between and and && (and between or and ||) is the precedence used to evaluate expressions.
Table 3.2 lists the highest-precedence operators first. The following conditions, which appear to do the same thing, are subtly but significantly different:
a or b and c a || b and c
In the former condition, the and takes precedence and is evaluated first. The overall condition is true if a is true or if both b and c are true.
In the latter condition, the || takes precedence, so c must be true, as must either a or b, to satisfy the condition.
Multiple Condition Branches
By using an else clause with an if statement, you can specify an alternate action to be taken if the condition is not met. The following example tests the value of $number and displays a message that says whether it is greater than or less than 10:
$number = 16;
if ($number < 10) {
echo "$number is less than ten";
}
else {
echo "$number is more than ten";
}
The else clause provides an either/or mechanism for conditional statements. To add more branches to a conditional statement, the elseif keyword can be used to add a further condition that is checked only if the previous condition in the statement fails.
The following example uses the date function to find the current time of day—date("H") gives a number between 0 and 23 that represents the hour on the clock—and displays an appropriate greeting:
$hour = date("H");
if ($hour < 12) {
echo "Good morning";
}
elseif ($hour < 17) {
echo "Good afternoon";
}
else {
echo "Good evening";
}
This code displays Good morning if the server time is between midnight and 11:59, Good afternoon from midday to 4:59 p.m., and Good evening from 5 p.m. onward.
Notice that the elseif condition only checks that $hour is less than 17 (5 p.m.). It does not need to check that the value is between 12 and 17 because the initial if condition ensures that PHP will not get as far as the elseif if $hour is less than 12.
The code in the else clause is executed if all else fails. For values of $hour that are 17 or higher, neither the if nor the elseif condition will be true.
The switch Statement
An if statement can contain as many elseif clauses as you need, but including many of these clauses can often create cumbersome code, and an alternative is available. switch is a conditional statement that can have multiple branches in a much more compact format.
The following example uses a switch statement to check $name against two lists to see whether it belongs to a friend:
switch ($name) {
case "Damon":
case "Shelley":
echo "Welcome, $name, you are my friend";
break;
case "Adolf":
case "Saddam":
echo "You are no friend of mine, $name";
break;
default:
echo "I do not know who you are, $name";
}
Each case statement defines a value for which the next block of PHP code will be executed. If you assign your first name to $name and run this script, you will be greeted as a friend if your name is Damon or Shelley, and you will be told that you are not a friend if your name is either Adolf or Saddam. If you have any other name, the script will tell you it does not know who you are.
There can be any number of case statements preceding the PHP code to which they relate. If the value that is being tested by the switch statement (in this case $name) matches any one of them, any subsequent PHP code will be executed until a break command is reached.
Any other value for $name will cause the default code block to be executed. As with an else clause, default is optional and supplies an action to be taken if nothing else is appropriate.