home | O'Reilly's CD bookshelfs | FreeBSD | Linux | Cisco | Cisco Exam  

Programming PHPProgramming PHPSearch this book

2.2. Data Types

PHP provides eight types of values, or data types. Four are scalar (single-value) types: integers, floating-point numbers, strings, and booleans. Two are compound (collection) types: arrays and objects. The remaining two are special types: resource and NULL. Numbers, booleans, resources, and NULL are discussed in full here, while strings, arrays, and objects are big enough topics that they get their own chapters (Chapter 4, Chapter 5, and Chapter 6).

2.2.1. Integers

Integers are whole numbers, like 1, 12, and 256. The range of acceptable values varies according to the details of your platform but typically extends from -2,147,483,648 to +2,147,483,647. Specifically, the range is equivalent to the range of the long data type of your C compiler. Unfortunately, the C standard doesn't specify what range that long type should have, so on some systems you might see a different integer range.

Integer literals can be written in decimal, octal, or hexadecimal. Decimal values are represented by a sequence of digits, without leading zeros. The sequence may begin with a plus (+) or minus (-) sign. If there is no sign, positive is assumed. Examples of decimal integers include the following:


Octal numbers consist of a leading 0 and a sequence of digits from 0 to 7. Like decimal numbers, octal numbers can be prefixed with a plus or minus. Here are some example octal values and their equivalent decimal values:

0755      // decimal 493
+010      // decimal 8

Hexadecimal values begin with 0x, followed by a sequence of digits (0-9) or letters (A-F). The letters can be upper- or lowercase but are usually written in capitals. Like decimal and octal values, you can include a sign in hexadecimal numbers:

0xFF        // decimal 255
0x10        // decimal 16
-0xDAD1     // decimal -56017

If you try to store a too-large integer in a variable, it will automatically be turned into a floating-point number.

Use the is_int( ) function (or its is_integer( ) alias) to test whether a value is an integer:

if (is_int($x)) {
    // $x is an integer

2.2.3. Strings

Because strings are so common in web applications, PHP includes core-level support for creating and manipulating strings. A string is a sequence of characters of arbitrary length. String literals are delimited by either single or double quotes:

'big dog'
"fat hog"

Variables are expanded within double quotes, while within single quotes they are not:

$name = "Guido";
echo "Hi, $name\n";       
echo 'Hi, $name';       
Hi, Guido
Hi, $name

Double quotes also support a variety of string escapes, as listed in Table 2-2.

Table 2-2. Escape sequences in double-quoted strings

Escape sequence

Character represented


Double quotes




Carriage return






Dollar sign


Left brace


Right brace


Left bracket


Right bracket

\0 through \777

ASCII character represented by octal value

\x0 through \xFF

ASCII character represented by hex value

A single-quoted string only recognizes \\ to get a literal backslash and \' to get a literal single quote:

$dos_path = 'C:\\WINDOWS\\SYSTEM';
$publisher = 'Tim O\'Reilly';
echo "$dos_path $publisher\n";

To test whether two strings are equal, use the == comparison operator:

if ($a == $b) { echo "a and b are equal" }

Use the is_string( ) function to test whether a value is a string:

if (is_string($x)) {
    // $x is a string

PHP provides operators and functions to compare, disassemble, assemble, search, replace, and trim strings, as well as a host of specialized string functions for working with HTTP, HTML, and SQL encodings. Because there are so many string-manipulation functions, we've devoted a whole chapter (Chapter 4) to covering all the details.

2.2.7. Resources

Many modules provide several functions for dealing with the outside world. For example, every database extension has at least a function to connect to the database, a function to send a query to the database, and a function to close the connection to the database. Because you can have multiple database connections open at once, the connect function gives you something by which to identify that connection when you call the query and close functions: a resource.

Resources are really integers under the surface. Their main benefit is that they're garbage collected when no longer in use. When the last reference to a resource value goes away, the extension that created the resource is called to free any memory, close any connection, etc. for that resource:

$res = database_connect( );   // fictitious function
$res = "boo";                // database connection automatically closed

The benefit of this automatic cleanup is best seen within functions, when the resource is assigned to a local variable. When the function ends, the variable's value is reclaimed by PHP:

function search ( ) {
  $res = database_connect( );

When there are no more references to the resource, it's automatically shut down.

That said, most extensions provide a specific shutdown or close function, and it's considered good style to call that function explicitly when needed rather than to rely on variable scoping to trigger resource cleanup.

Use the is_resource( ) function to test whether a value is a resource:

if (is_resource($x)) {
    // $x is a resource

Library Navigation Links

Copyright © 2003 O'Reilly & Associates. All rights reserved.