Learning PHP, MySQL, JavaScript, and CSS, 2nd Edition

Conditionals

Conditionals alter program flow. They enable you to ask questions about certain things and respond to the answers you get in different ways. Conditionals are central to dynamic web pages—the goal of using PHP in the first place—because they make it easy to create different output each time a page is viewed.

There are three types of nonlooping conditionals: the if statement, the switch statement, and the ? operator. By nonlooping, I mean that the actions initiated by the statement take place and program flow then moves on, whereas looping conditionals (which we’ll come to shortly) execute code over and over until a condition has been met.

The if Statement

One way of thinking about program flow is to imagine it as a single-lane highway that you are driving along. It’s pretty much a straight line, but now and then you encounter various signs telling you where to go.

In the case of an if statement, you could imagine coming across a detour sign that you have to follow if a certain condition is TRUE. If so, you drive off and follow the detour until you rejoin your original route; you then continue on your way in your original direction. Or, if the condition isn’t TRUE, you ignore the detour and carry on driving (see Figure 4-1).

Figure 4-1. Program flow is like a single-lane highway

The contents of the if condition can be any valid PHP expression, including equality, comparison, tests for zero and NULL, and even the values returned by functions (either built-in functions or ones that you write).

The action to take when an if condition is TRUE are generally placed inside curly braces, {}. You can omit the braces if you have only a single statement to execute, but if you always use curly braces you’ll avoid potentially difficult-to-trace bugs, such as when you add an extra line to a condition but forget to add the braces in, so it doesn’t get evaluated. (Note that for reasons of layout and clarity, many of the examples in this book ignore this suggestion and omit the braces for single statements.)

In Example 4-19, imagine that it is the end of the month and all your bills have been paid, so you are performing some bank account maintenance.

Example 4-19. An if statement with curly braces

<?php
if ($bank_balance < 100)
{
    $money        = 1000;
    $bank_balance += $money;
}
?>

In this example, you are checking your balance to see whether it is less than 100 dollars (or whatever your currency is). If so, you pay yourself 1000 dollars and then add it to the balance. (If only making money were that simple!)

If the bank balance is 100 dollars or greater, the conditional statements are ignored and program flow skips to the next line (not shown).

In this book, opening curly braces generally start on a new line. Some people like to place the first curly brace to the right of the conditional expression instead. Either of these approaches is fine, because PHP allows you to set out your whitespace characters (spaces, newlines, and tabs) any way you choose. However, you will find your code easier to read and debug if you indent each level of conditionals with a tab.

The else Statement

Sometimes when a conditional is not TRUE, you may not want to continue on to the main program code immediately but might wish to do something else instead. This is where the else statement comes in. With it, you can set up a second detour on your highway, as in Figure 4-2.

Figure 4-2. The highway now has an if detour and an else detour

What happens with an if...else statement is that the first conditional statement is executed if the condition is TRUE, but if it’s FALSE, the second one is executed. One of the two choices must be executed. Under no circumstances can both (or neither) be executed. Example 4-20 shows the use of the if...else structure.

Example 4-20. An if…else statement with curly braces

<?php
if ($bank_balance < 100)
{
    $money        = 1000;
    $bank_balance += $money;
}
else
{
    $savings      += 50;
    $bank_balance -= 50;
}
?>

In this example, having ascertained that you have over $100 in the bank, the else statement is executed, by which you place some of this money into your savings account.

As with if statements, if your else has only one conditional statement, you can opt to leave out the curly braces. (Curly braces are always recommended, though: they make the code easier to understand, and they let you easily add more statements to the branch later.)

The elseif Statement

There are also times when you want a number of different possibilities to occur, based upon a sequence of conditions. You can achieve this using the elseif statement. As you might imagine, it is like an else statement, except that you place a further conditional expression prior to the conditional code. In Example 4-21, you can see a complete if...elseif...else construct.

Example 4-21. An if…elseif…else statement with curly braces

<?php
if ($bank_balance < 100)
{
    $money        = 1000;
    $bank_balance += $money;
}
elseif ($bank_balance > 200)
{
    $savings      += 100;
    $bank_balance -= 100;
}
else
{
    $savings      += 50;
    $bank_balance -= 50;
}
?>

In this example, an elseif statement has been inserted between the if and else statements. It checks whether your bank balance exceeds $200 and, if so, decides that you can afford to save $100 of it this month.

Although I’m starting to stretch the metaphor a bit too far, you can imagine this as a multiway set of detours (see Figure 4-3).

Figure 4-3. The highway with if, elseif, and else detours

Note

An else statement closes one of the following: an if...else statement or an if...elseif...else statement. You can leave out a final else if it is not required, but you cannot have one before an elseif; neither can you have an elseif before an if statement.

You may have as many elseif statements as you like, but as the number of elseif statements increases it becomes advisable to consider a switch statement instead, if it fits your needs. We’ll look at that next.

The switch Statement

The switch statement is useful in cases in which one variable or the result of an expression can have multiple values, which should each trigger a different function.

For example, consider a PHP-driven menu system that passes a single string to the main menu code according to what the user requests. Let’s say the options are Home, About, News, Login, and Links, and we set the variable $page to one of these, according to the user’s input.

The code for this written using if...elseif...else might look like Example 4-22.

Example 4-22. A multiple-line if…elseif…statement

<?php
if     ($page == "Home")  echo "You selected Home";
elseif ($page == "About") echo "You selected About";
elseif ($page == "News")  echo "You selected News";
elseif ($page == "Login") echo "You selected Login";
elseif ($page == "Links") echo "You selected Links";
?>

Using a switch statement, the code might look like Example 4-23.

Example 4-23. A switch statement

<?php
switch ($page)
{
    case "Home":
        echo "You selected Home";
        break;
    case "About":
        echo "You selected About";
        break;
    case "News":
        echo "You selected News";
        break;
    case "Login":
        echo "You selected Login";
        break;
    case "Links":
        echo "You selected Links";
        break;
}
?>

As you can see, $page is mentioned only once at the start of the switch statement. Thereafter, the case command checks for matches. When one occurs, the matching conditional statement is executed. Of course, in a real program you would have code here to display or jump to a page, rather than simply telling the user what was selected.

Note

One thing to note about switch statements is that you do not use curly braces inside case commands. Instead, they commence with a colon and end with the break statement. The entire list of cases in the switch statement is enclosed in a set of curly braces, though.

Breaking out

If you wish to break out of the switch statement because a condition has been fulfilled, use the break command. This command tells PHP to break out of the switch and jump to the following statement.

If you were to leave out the break commands in Example 4-23 and the case of “Home” evaluated to be TRUE, all five cases would then be executed. Or if $page had the value “News,” all the case commands from then on would execute. This is deliberate and allows for some advanced programming, but generally you should always remember to issue a break command every time a set of case conditionals has finished executing. In fact, leaving out the break statement is a common error.

Default action

A typical requirement in switch statements is to fall back on a default action if none of the case conditions are met. For example, in the case of the menu code in Example 4-23, you could add the code in Example 4-24 immediately before the final curly brace.

Example 4-24. A default statement to add to Example 4-23

    default: echo "Unrecognized selection";
        break;

Although a break command is not required here because the default is the final substatement, and program flow will automatically continue to the closing curly brace, should you decide to place the default statement higher up it would definitely need a break command to prevent program flow from dropping into the following statements. Generally, the safest practice is to always include the break command.

Alternative syntax

If you prefer, you may replace the first curly brace in a switch statement with a single colon and the final curly brace with an endswitch command, as in Example 4-25. However, this approach is not commonly used and is mentioned here only in case you encounter it in third-party code.

Example 4-25. Alternate switch statement syntax

<?php
switch ($page):
    case "Home":
        echo "You selected Home";
        break;

    // etc...

    case "Links":
        echo "You selected Links";
        break;
endswitch;
?>

The ? Operator

One way of avoiding the verbosity of if and else statements is to use the more compact ternary operator, ?, which is unusual in that it takes three operands rather than the more usual two.

We briefly came across this in Chapter 3 in the discussion about the difference between the print and echo statements, as an example of an operator type that works well with print but not echo.

The ? operator is passed an expression that it must evaluate, along with two statements to execute: one for when the expression evaluates to TRUE, the other for when it is FALSE. Example 4-26 shows code we might use for writing a warning about the fuel level of a car to its digital dashboard.

Example 4-26. Using the ? operator

<?php
echo $fuel <= 1 ? "Fill tank now" : "There's enough fuel";
?>

In this statement, if there is one gallon or less of fuel (in other words, if $fuel is set to 1 or less), the string “Fill tank now” is returned to the preceding echo statement. Otherwise, the string “There’s enough fuel” is returned. You can also assign the value returned in a ? statement to a variable (see Example 4-27).

Example 4-27. Assigning a ? conditional result to a variable

<?php
$enough = $fuel <= 1 ? FALSE : TRUE;
?>

Here, $enough will be assigned the value TRUE only when there is more than a gallon of fuel; otherwise, it is assigned the value FALSE.

If you find the ? operator confusing, you are free to stick to if statements, but you should be familiar with it because you’ll see it in other people’s code. It can be hard to read, because it often mixes multiple occurrences of the same variable. For instance, code such as the following is quite popular:

$saved = $saved >= $new ? $saved : $new;

If you take it apart carefully, you can figure out what this code does:

$saved =                    // Set the value of $saved
        $saved >= $new      // Check $saved against $new
    ?                       // Yes, comparison is true ...
        $saved              // ... so assign the current value of $saved
    :                       // No, comparison is false ...
        $new;               // ... so assign the value of $new

It’s a concise way to keep track of the largest value that you’ve seen as a program progresses. You save the largest value in $saved and compare it to $new each time you get a new value. Programmers familiar with the ? operator find it more convenient than if statements for such short comparisons. When not used for writing compact code, it is typically used to make some decision inline, such as when testing whether a variable is set before passing it to a function.