Conditional
Statements
Conditional Statements allow you to branch the path of execution in a
script based on whether a single, or multiple conditions, evaluate to
true or false. Put simply, they let you test things and perform various
actions based on the results.
If Statements
Take a look at the following:
<?php
$x=1;
if ($x == 1) print '$x is equal to 1';
?> |
This example illustrates the
simplest kind of If Statement. If Statements always begin with "if", followed
by a condition surrounded in parentheses. If the condition evaluates to
true, then the statement or statements immediately following the condition
will be executed. In this case, had the condition been false, nothing would
have occurred and you would have seen a blank browser window when the script
was run.
When you have more than one statement to be executed within a control structure,
it's necessary to surround them with brackets:
|
<?php
$x=1;
if ($x
== 1) {
print '$x is equal to 1';
$x++;
print
'now $x is equal to 2';
}
?>
|
Keep in mind that the positioning of the elements does not affect the execution
of the script. All of the example arrangements below are perfectly valid
not only for If Statements, but for every form of control loop.
if
($x == 1) print '$x is equal to 1';
if ($x == 1)
print '$x is equal to 1';
if ($x == 1) { print '$x is equal to 1'; }
if ($x == 1) {
print '$x is equal to 1';
} |
In the interests of clarity, many programmers opt to use indenting and
brackets even on one-line blocks of code; however, it is ultimately a
matter of personal coding preference.
You can also include multiple conditions within parentheses. For the nested
statements to execute, all of the conditions must evaluate to true.
<?php
$x=1;
if ($x == 1 OR $x == 2) print '$x is equal to 1 (or maybe 2)';
?> |
Else Statements
As the name implies, Else
Statements allow you to do something else if the condition within
an If Statement evaluated to false:
<?php
$x=1;
if ($x == 2) {
print '$x is equal to 2';
} else {
print '$x is equal to 1';
}
?> |
Else If Statements
Thus far, we have been able to respond to one condition, and do something
if that condition is not true. But what about evaluating multiple conditions?
You could use a series of If Statements to test each potential condition,
but in some situations that is not a suitable option. Here is where Else
If Statements come in.
A combination of If and Else Statements, Else If Statements are evaluated
sequentially if the condition within the If Statement is false. When a
condition within an Else If Statement evaluates to true, the nested statements
are parsed, the script stops executing the entire If/Else if/Else Structure.
The rest of the script proceeds to be parsed.
Take a look at the following example:
|
<?php
$x=1;
if ($x == 2) {
print '$x is equal to 2';
} else if ($x == 1) {
print '$x is equal to 1';
} else {
print '$x does not equal 2 or 1';
}
?>
|
The final else statement can be left off if you do not want anything to
happen if none of the If or Else If Statements are true:
|
<?php
$x=0;
if ($x == 2) {
print '$x is equal to 2';
} else if ($x == 1) {
print '$x is equal to 1';
}
?>
|
In this case, since neither the condition within the If or Else if Conditions
are true, and no Else Statement was provided, nothing would be outputted
to the browser.
Switches
Switches are a good alternative to If/Else if/Else Statements in situations
where you want to check multiple values against a single variable or condition.
This is the basic syntax:
|
<?php
$var = "yes";
switch ($var) {
case "yes":
print '$var is equal to yes';
break;
case "no":
print
'$var is equal to no';
break;
}
?>
|
After running this code snippet,
much of what is here will probably make sense to you. In the first line
of the switch statement, we have the identifier "switch" followed by a
variable surrounded by parenthesis. Each case includes a possible value
for the variable.
Switches execute a little differently than If/Else if/Else statements.
Once a case value matches the value of the switch expression, every following
statement is executed, including those following other cases.
To prevent this from happening, a break statement is used. "Break;" ends
the execution of the switch statement, and lets the script continue execution;
it can also be used in while or for loops.
Optionally, you may also include a special case called "default".
This case works much like and Else Statement, and will execute if all
the other cases are found to be false. This case should be the very last
one you include.
|
<?php
$var = "yes";
switch ($var) {
case "maybe":
print '$var is equal to yes';
break;
case "no":
print
'$var is equal to no';
break;
default:
print 'none of the other two cases
were true, so this sentance will be printed out instead.';
}
?>
|
Similar to the Break Statement is the Exit Statement. Exit is particularly
useful in situations where you run into what would be considered a "fatal
error" (for example, if the user had entered a password that was
incorrect) or any other time you needed to end the execution of a script
before it naturally terminated.
<?php
$var = "yes";
switch ($var) {
case "yes":
print '$var is equal to yes';
exit;
case "no":
print '$var is equal to no';
break;
}
print "this will not be printed, because the script will have
terminate before this line is reached";
?> |
Unlike break, exit may be used anywhere in your scripts, inside or outside
of control structures.
The Ternary Operator
Though Technically an Operator, not a Control Structure, the Ternary Operator,
represented by "?", can be used as shorthand for simple If/Else Statements.
It can only be used in situations where you want execute a single expression
based on whether a single condition is true or false:
<?php
$x = 1;
($x==1) ? (print '$x is equal to 1') : (print '$x is not equal to
1');
?> |
The condition is contained
within the first set of parentheses. If it evaluates to true, then the
expression within the second set of parentheses will be performed. Otherwise,
the expression in the third set will be performed:
| (condition)
? (executes if the condition is true) : (executes if the condition
is false); |
|
