Web Database Applications With Php And Mysql (2nd Edition) [Electronic resources] نسخه متنی

2.2 Conditions and Branches

Conditionals add control to
scripts and permit choices. Different statements are executed
depending on whether expressions are true or
false. There are two branching statements in PHP:
if, with the optional else
clause, and switch, usually with two or more
case clauses.

2.2.1 if...else Statement

The if

statement conditionally controls
execution. The basic format of an if statement is
to test whether a condition is true and, if so, to
execute one or more statements.

The following if statement executes the
print statement and outputs the string when the
conditional expression, $var is greater than 5, is
true:

if ($var > 5)
print "The variable is greater than 5";

The expressions used in the examples in this section compare integers. They can be used to compare strings but usually not with the expected results. If strings need to be compared, use the PHP string library function strcmp( ). It's discussed in more detail in Chapter 3.

Multiple statements can be executed as a block by encapsulating the
statements within braces. If the expression evaluates as
true, the statements within the braces are
executed. If the expression isn't
true, none of the statements are executed.
Consider an example in which three statements are executed if the
condition is true:

if ($var > 5)
{
print "The variable is greater than 5.";
// So, now let's set it to 5
$var = 5;
print "In fact, now it is equal to 5.";
}

Without the braces, an if statement executes only
the single, immediately following statement when the conditional
expression evaluates to true.

The if statement can have an optional
else
clause to execute a statement or block
of statements if the expression evaluates as
false. Consider an example:

if ($var > 5)
print "Variable greater than 5";
else
print "Variable less than or equal to 5";

It's also common for the else
clause to execute a block of statements in braces, as in this
example:

if ($var < 5)
{
print "Variable is less than 5";
print "-----------------------";
} 
else
{
print "Variable is equal to or larger than 5";
print "-------------------------------------";
}

Consecutive conditional tests can lead to examples such as:

if ($var < 5)
print "Value is very small";
else
if ($var < 10)
print "Value is small";
else
if ($var < 20)
print "Value is normal";
else
if ($var < 30)
print "Value is big";
else
print "Value is very big";

The indentation in the preceding example highlights the nested nature
of the multiple tests. If consecutive, cascading tests are needed,
the elseif statement can be used. The choice of
which method to use is a matter of personal preference. This example
has the same functionality as the previous example:

if ($var < 5)
print "Value is very small";
elseif ($var < 10)
print "Value is small";
elseif ($var < 20)
print "Value is normal";
elseif ($var < 30)
print "Value is big";
else
print "Value is very big";

2.2.2 switch Statement

The switch
statement can be used as an
alternative to if to select an option from a list
of choices. The following example executes different code for
different integer values, or cases of the variable
$menu. A case clause is
provided for values 1, 2, 3, and 4, with a
default: case provided for all other values:

switch ($menu)
{
case 1:
print "You picked one";
break;
case 2:
print "You picked two";
break;
case 3:
print "You picked three";
break;
case 4:
print "You picked four";
break;
default:
print "You picked another option";
}

This example can be implemented with if and
elseif, but the switch method
is usually more compact, readable, and easier to type. The use of
break statements is important: they prevent
execution of statements that follow in the switch
statement and force execution to jump to the statement that follows
the closing brace.

If break statements are omitted from a
switch statement, you can get an unexpected
result. For example, without the break statements,
if the user chooses option 3, the script outputs:

 You picked three. You picked four. You picked another option

These results are often a source of difficult-to-detect bugs;
however, by intentionally omitting the break
statement, you can group cases together as shown in the following
switch statement:

$score = "Distinction";
switch ($score)
{
case "High Distinction":
case "Distinction":
print "Good student";
break;
case "Credit":
case "Pass":
print "Average student";
break;
default:
print "Poor student";
}

While not mandatory, the default: case is useful
when default processing is performed on all but selected special
cases, or to handle unexpected values when expected values have
corresponding cases.

2.2.3 Conditional Expressions

Now
we'll look at what can go inside the parentheses of
an if statement, and other control statements. The
most common conditional comparison is to test the equality or
inequality of two expressions. Equality is checked with the
double-equal operator,
==

; if the value on the left-hand side
is equal to the value on the right-hand side, then the expression
evaluates to true. The expression
($var == 3)
in the following example evaluates to true:

$var = 3;
if ($var == 3)
print "Equals 3";

Inequality is tested with the not-equals operator,
!=

. Both
evaluate to a Boolean result of true or
false.

If the equality operator == and the assignment operator = are unfamiliar, beware: they are easy to inadvertently interchange. This is a very common bug and hard to detect. The value of the conditional expression ($var = 1) evaluates as true, because the expression takes its value from the value on the right hand side of the assignment operator; in this case 1. Here is an example of a common mistake, which overwrites the original value of the variable and always prints the statement: if ($var = 1) print "Variable equals 1."; The error of incorrectly replacing an assignment with == is a far less common mistake. However, it's also difficult to detect because an incorrectly written assignment of $var == 1; is quietly evaluated as true or false with no effect on $var.

Expressions can be combined with parentheses and with the Boolean
operators && (and) and
|| (or). For example, the following expression
returns true and prints the message if $var is equal to 3 or $var2 is
equal to 7:

if (($var == 3) || ($var2 == 7))
print "Equals 3 or 7";

The following expression returns true and prints
the message if $var equals 2 and
$var2 equals 6:

if (($var == 2) && ($var2 == 6))
print "The variables are equal to 2 and 6";

Interestingly, if the first part of the expression
($var == 2)
evaluates as false, PHP doesn't
evaluate the second part of the expression ($var2
== 6), because the overall
expression can never be true; both conditions must
be true for an && (and)
operation to be true. Similarly, in the previous
example, if ($var ==
3), then there's no need to check
if ($var2 ==
7).

This
short-circuit
evaluation property has
implications for design; to speed code, write the expression most
likely to evaluate as false as the left-most
expression, and ensure that computationally expensive operations are
as right-most as possible.

Never assume that expressions combined with the Boolean operators && and || are evaluated. PHP uses short-circuit evaluation when determining the result of a Boolean expression.

Conditional expressions can be negated with the Boolean not operator
!. The following example shows how an expression that tests if
$var is equal to 2 or 6 is negated:

if (($var == 2) || ($var == 6))
print "The variable var is equal to 2 or 6";
if (!(($var == 2) || ($var == 6)))
print "The variable var is not equal to 2 or 6";

Unlike the && and ||
operators, ! works on a single value as the
following example highlights:

// Set a Boolean variable
$found = false;
// The following message is printed
if (!$found)
print "Expression is true";

More complex expressions can be formed through combinations of the
Boolean operators and the liberal use of parentheses. For example,
the following expression evaluates as true and
prints the message if one of the following is
true: $var equals 6 and
$var2 equals 7, or $var equals
4 and $var2 equals 1.

if ((($var == 6) && ($var2 == 7)) || (($var == 4) && ($var2 == 1)))
print "Expression is true";

As in assignment expressions, parentheses ensure that evaluation
occurs in the required order.