PHP Using Variables and Operators Mohammed M. Hassoun 2012

Using Variables and Operators

Storing Data in Variables A variable is simply a container that’s used to store both numeric and non-numeric information. And just as with any container, you can move it from place to place, add stuff to it, or empty it out on the floor in a pile and fill it with something completely different. To stretch the analogy a little further, just as it’s a good idea to label every container, so too should you give every variable in your program a name. As a general rule, these names should make sense and should be easy to understand. In the real world, this makes it easier to find things; in the programming world, it makes your code cleaner and easier to understand by others. As someone who’s been there, I can tell you that there’s nothing more frustrating than spending three hours searching a bunch of boxes for Mom’s best china, only to realize it’s in the one marked “Miscellaneous” together with an old rubber bone and some crumbly biscuits!

PHP has some simple rules for naming variables. Every variable name must be preceded with a dollar ($) symbol and must begin with a letter or underscore character, optionally followed by more letters, numbers, or underscore characters. Common punctuation characters, such as commas, quotation marks, or periods, are not permitted in variable names; neither are spaces. So, for example, $root, $_num, and $query2 are all valid variable names, while $58%, $1day, and are all invalid variable names.

Understanding PHP’s Data Types The values assigned to a PHP variable may be of different data types, ranging from simple string and numeric types to more complex arrays and objects. You’ve already seen two of these, strings and numbers, in action in previous examples. Here’s a full-fledged example, which introduces three more data types: <?php // Boolean $validUser = true; // integer $size = 15; // floating point $temp = 98.6; // string $cat = 'Siamese'; // null $here = null; ?>

● Booleans are the simplest of all PHP data types. Like a switch that has only two states, on and off, it consists of a single value that may be set to either 1 (true) or 0 (false). In this listing, $validUser is a Boolean variable set to true. ● PHP also supports two numeric data types: integers and floating-point values. Floating-point values (also known as floats or doubles) are decimal or fractional numbers, while integers are round numbers. Both may be less than, greater than, or equal to zero. In this listing, $size holds an integer value, while $temp holds a floating-point value. ● For non-numeric data, PHP offers the string data type, which can hold letters, numbers, and special characters. String values must be enclosed in either single quotes or double quotes. In the previous listing, $cat is a string variable containing the value 'Siamese'. ● You may also encounter the NULL data type, which is a “special” data type first introduced in PHP 4. NULLs are used to represent “empty” variables in PHP; a variable of type NULL is a variable without any data. In the preceding listing, $here is NULL.

Manipulating Variables with Operators By themselves, variables are simply containers for information. In order to do anything useful with them, you need operators. Operators are symbols that tell the PHP processor to perform certain actions. For example, the addition (+) symbol is an operator that tells PHP to add two variables or values, while the greater-than (>) symbol is an operator that tells PHP to compare two values. PHP supports more than 50 such operators, ranging from operators for arithmetical operations to operators for logical comparison and bitwise calculations. This section discusses the most commonly used operators.

Performing Arithmetic Operations PHP supports all standard arithmetic operations, as illustrated by the list of operators in Table 2-2.

Concatenating Strings To combine strings, use PHP’s concatenation operator, which happens to be a period (.). The following example illustrates: <?php // define variables $country = 'England'; $city = 'London'; // combine into single string // output: 'Welcome to London, the coolest city in all of England' echo 'Welcome to '. $city. ', the coolest city in all of '. $country; ?>

Comparing Variables PHP lets you compare one variable or value with another via its wide range of comparison operators, listed in Table 2-3. It’s worth making special mention here of the === operator. This operator allows for stricter comparison between variables: it only returns true if the two variables or values being compared hold the same information and are of the same data type.

Performing Logical Tests When building complex conditional expressions a topic that will be discussed in detail in Chapter 3 you will often come across situations where it’s necessary to combine one or more logical tests. PHP’s three most commonly used logical operators, listed in Table 2-4, are intended specifically for this situation.

Other Useful Operators There are a few other operators that tend to come in handy during PHP development. First, the addition-assignment operator, represented by the symbol +=, lets you simultaneously add and assign a new value to a variable.

Understanding Operator Precedence Back in math class, you probably learned about BODMAS, a mnemonic that specifies the order in which a calculator or a computer performs a sequence of mathematical operations: Brackets, Order, Division, Multiplication, Addition, and Subtraction. Well, PHP follows a similar set of rules when determining which operators have precedence over others—and learning these rules can save you countless frustrating hours debugging a calculation that looks right, yet somehow always returns the wrong result!.The following list (a short version of a much longer list in the PHP manual) illustrates PHP’s most important precedence rules. Operators at the same level have equal precedence. ● ● ! ● * / % ● + -. ● >= ● == != === !== ● && ● || ● = += -= *= /=.= %= &= |= ^=

Precedence Rules

Handling Form Input So far, all the examples you’ve seen have had their variables neatly defined at the top of the script listing. However, as your PHP scripts become more complex, this happy situation will change, and you’ll need to learn how to interact with user-supplied input. The most common source of this information is a Web form, and PHP comes with a simple mechanism to retrieve information submitted through such forms. To illustrate, consider the following simple Web form (choose.html), which asks you to select a brand of automobile and enter your desired color: Type: Porsche 911 Volkswagen Beetle Ford Taurus Color:

As forms go, this is a fairly simple one: it has a selection list and an input box. Figure 2-2 illustrates what it looks like. Notice the 'action' attribute of this Web form: it references a PHP script named car.php. This is the script that receives the data entered into the form once the form is submitted. Notice also the form’s 'method' attribute, which specifies that the submission of data will occur through the POST method. With these two facts firmly in mind, let’s take a look at car.php next:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "DTD/xhtml1-transitional.dtd"> Success! <?php // get form input $type = $_POST['selType']; $color = $_POST['txtColor']; // use form input echo "Your $color $type is ready. Safe driving!"; ?>

What’s going on here? Well, whenever a form is submitted to a PHP script through the POST method, the form’s input variables and their values automatically become available to the PHP script through a special container variable named $_POST. Accessing the value entered into a particular form field then becomes as simple as referencing $_POST with the corresponding field’s name, as in the preceding script. Consider, for example, the task of accessing the color entered by the user in the Web form. From the form code, it can be seen that the text input field designated for this data in the form is named 'txtColor'. Therefore, within the PHP script, the value entered into this text input field can be accessed using the syntax $_POST['txtColor'].

This value can then be used in the normal fashion: it may be printed to the Web page, assigned to another variable, or manipulated using one of the many operators discussed in preceding sections. Figure 2-3 illustrates the result of submitting the form. In case your Web form submits data using the GET method instead of the POST method, PHP has you covered there as well: form input submitted using the GET method finds a home in the $_GET container variable and may be accessed by referencing $_GET instead of $_POST.

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