PHP's Building Blocks: Data Types, Literals, Variables, and Constants
- 4.1 Data Types
- 4.2 Variables
- 4.3 Constants
- 4.4 Chapter Summary
- Chapter 4 Lab
This chapter is from the book
This chapter is from the book
This chapter is from the book
)
"One man's constant is another man's variable."
—Alan Perlis
4.1 Data Types
A program can do many things, including perform calculations, sort names, prepare phone lists, display images, play chess, ad infinitum. To do anything, however, the program works with the data that is given to it. Data types specify what kind of data, such as numbers and characters, can be stored and manipulated within a program. PHP supports a number of fundamental basic data types, such as integers, floats, and strings. Basic data types are the simplest building blocks of a program. They are called scalars and can be assigned a single literal value such as a number, 5.7, or a string of characters, such as "hello", a date and time, or a boolean (true/false). See Figure 4.1.
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Figure 4.1 Scalars hold one value.
PHP also supports composite data types, such as arrays and objects. Composite data types represent a collection of data, rather than a single value (see Figure 4.2). The composite data types are discussed in Chapter 8, "Arrays," and Chapter 17, "Objects."
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Figure 4.2 Arrays and objects hold multiple values.
The different types of data are commonly stored in variables. Examples of PHP variables are $num = 5 or $name = "John" where variables $num and $name are assigned an integer and a string, respectively. Variables hold values that can change throughout the program, whereas once a constant is defined, its value does not change. PHP_VERSION and PHP_OS are examples of predefined PHP constants. The use of PHP variables and constants is addressed in "Variables" on page 70 and "Constants" on page 99 of this chapter.
PHP supports four core data types:
- Integer
- Float (also called double)
- String
- Boolean
In addition to the four core data types, there are four other special types:
- Null
- Array
- Object
- Resources
4.1.1 Numeric Literals
PHP supports both integers and floating-point numbers. See Example 4.1.
- Integers— Integers are whole numbers and do not contain a decimal point; for example, 123 and –6. Integers can be expressed in decimal (base 10), octal (base 8), and hexadecimal (base 16), and are either positive or negative values.
- Floating-point numbers— Floating-point numbers, also called doubles or reals, are fractional numbers such as 123.56 or –2.5. They must contain a decimal point or an exponent specifier, such as 1.3e–2. The letter "e" can be either upper or lowercase.
PHP numbers can be very large (the size depends on your platform), but a precision of 14 decimal digits is a common value or (~1.8e308).
Example 4.1.
12345 integer 23.45 float .234E–2 float in scientific notation .234e+3 float in scientific notation 0x456fff integer in base 16, hexadecimal 0x456FFF integer in base 16, hexadecimal 0777 integer in base 8, octal
Example 4.2.
<html>
<head><title>Printing Numbers</title>
</head>
<body bgcolor="lightblue">
<font face = "arial" size = '+1'>
<?php
print "The positive integer is <em><b>" . 5623 . "
.</b></em><br />";
print "The negative integer is <em><b>" . -22 . ".</b></em><br />";
print "The floating point number is <em><b>" . 15.3 . "
.</b></em><br />";
print "The number in scientfic notation is <em><b> " . 5e3 . "
. </b></em><br />";
print "\tThe string is: <em><b>I can't help you!</em>
</b><br />";
?>
</body>
</html>
)
Figure 4.3 Output from Example 4.2.
4.1.2 String Literals and Quoting
We introduce strings in this chapter but Chapter 6, "Strings," provides a more comprehensive coverage. String literals are a row of characters enclosed in either double or single quotes.1 The quotes must be matched. If the string starts with a single quote, it must end with a matching single quote; likewise if it starts with a double quote, it must end with a double quote. If a string of characters is enclosed in single quotes, the characters are treated literally (each of the characters represents itself). We can say the single quotes are the democratic quotes: All characters are treated equally.
Double quotes do not treat all characters equally. If a string is enclosed in double quotes, most of the characters represent themselves, but dollar signs and backslashes have a special meaning as shown in the following examples.
Single quotes can hide double quotes, and double quotes can hide single quotes:2
"This is a string" 'This is another string' "This is also 'a string'" 'This is "a string"'
An empty set of quotes is called the null string. If a number is enclosed in quotes, it is considered a string; for example, "5" is a string, whereas 5 is a number.
Strings are called constants or literals. The string value "hello" is called a string constant or literal. To change a string requires replacing it with another string.
Strings can contain escape sequences (a single character preceded with a backslash). Escape sequences cause a character to behave in a certain way; for example, a "\t" represents a tab and "\n" represents a newline. The backslash is also used for quoting a single character so that it will not be interpreted; for example, \$5.00 where the dollar sign in PHP is used to represent variables rather than money. \$5.00 could also be written as '$5' because single quotes protect all characters from interpretation.
Here documents, also called here-docs, provide a way to create a block of text that simplifies writing strings containing lots of single quotes, double quotes, and variables (see Example 4.4).
Example 4.3.
<html>
<head><title>Quotes</title></head>
<body bgcolor="lightblue"><font size='+1'>
1 <?php
2 $name = "Nancy"; // Setting a PHP variable
print "<ol>";
3 print "<li> $name is my friend.</li>"; // Double quotes
4 print '<li> $name is my neighbor.</li>'; // Single quotes
5 print "<li> I can't go with you.</li>"; // Nested quotes
6 print "<li> She cried, \"Help!\"</li>"; // Escaping quotes
7 print "<li> I need \$5.00.</li>"; // The backslash
// quotes one character
8 print "<li> $name needs ". '$5.00 </li>'; // Nested quotes
print "</ol>";
?>
</body>
</html>
Explanation
|
1 |
PHP program starts here. |
|
2 |
$name is a PHP variable. It is assigned the string "Nancy". You will learn all about variables in the section "Variables" on page 70. |
|
3 |
When a string is enclosed within double quotes, the PHP interpreter will substitue the variable with its value; for example, $name will be replaced with "Nancy". |
|
4 |
When a string is enclosed in single quotes, all characters are treated as literals. Variable substitution will not occur. |
|
5 |
Single quotes can be nested within double quotes and vice versa. |
|
6 |
Quotes can be escaped with a backslash to make them literal characters within a string. |
|
7 |
The dollar sign is escaped from PHP interpretation, that is, is treated as a literal character. |
|
8 |
A string in double quotes is concatenated to a string in single quotes. Just as the backslash protects the dollar sign from interpretaion, so do the single quotes. Remember, characters in single quotes are all treated as literals; that is, PHP does not consider any of the enclosed characters as special. See the output in Figure 4.4. |
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Figure 4.4 Single and double quotes.
The Here Document—A Special Kind of Quoting
Here documents are a kind of quoting popular in a number of languages, such as JavaScript, Perl, Shell scripts, and so on. Here documents, also called here-docs, allow you to quote a large block of text within your script without using multiple print statements and quotes. The entire block of text is treated as though it is surrounded by double quotes. This can be useful if you have a large block of HTML within your PHP script interspersed with variables, quotes, and escape sequences.
Rules for a Here Document:
- The user-defined delimiter word starts and terminates the here document. Text is inserted between the delimiters. The delimiter can contain letters, numbers, and the underscore character. The first letter must be a letter or an underscore. By convention, the delimiter should be in all uppercase letters to make it stand out from other words in your script. The delimeter is preceded by three < characters; for example, <<<DELIMITER
print <<<HERE_DOC_DELIMITER <text here> ... < more text> ... HERE_DOC_DELIMITER
- The delimiter cannot be surrounded by any spaces, comments, or other text. The final delimiter can optionally be terminated with a semicolon and must be on a line by itself.
- All variable and escape sequences will be interpreted within the here document.
Example 4.4.
1 <?php
2 $bgcolor="darkblue";
$tablecolor = "yellow";
3 print <<< MY_BOUNDARY
4 <html><head><title>heredoc</title></head>
5 <body bgcolor="$bgcolor">
6 <table border="1" bgcolor=$tablecolor>
<tr><th>Author</th<th>Book</th></tr>
<tr>
<td>Marcel Proust</td>
<td>Remembrance of Things Past</td>
</tr>
<tr>
<td>Charles Dickens</td>
<td>Tale of Two Cities</td>
</tr>
</table>
</body>
7 </html>
8 MY_BOUNDARY;
?>
Explanation
|
1 |
PHP starts here. |
|
2 |
Two scalar variables are defined. |
|
3 |
This is the here-doc. The user-defined terminator, MY_BOUNDARY, is prepended with <<<. There can be no space after the terminator; otherwise an error like this will be displayed: Parse error: syntax error, unexpected T_SL in c:\wamp\www\exemples\ch4variables\heredoc.php on line 4 |
|
4 |
All of the HTML document is embedded in the here document. The HTML will be sent to the browser as is. Any PHP code embedded withing the HTML tags will be handled by the PHP interpreter. |
|
5 |
The value of the variable, $bgcolor, will be assigned as the background color of the page. |
|
6 |
An HTML table is started here. The value of the variable, $tablecolor, will be assigned as the background color of the table cells. |
|
7 |
The HTML document ends here, inside the here-doc. |
|
8 |
The user-defined terminator, MY_BOUNDARY, marks the end of the here document. There can be no spaces surrounding the terminator. The semicolon is optional. |
)
Figure 4.5 Here document output.
Escape Sequences
Escape sequences consist of a backslash followed by a single character. When enclosed in double quotes, the backslash causes the interpretation of the next character to "escape" from its normal ASCII code and to represent something else (see Table 4.1). To display the escape sequences in your browser, the HTML <pre> tag can be used (see Example 4.5); otherwise, the escape sequences placed within your PHP script will not be interpreted.
Table 4.1. Escape Sequences
|
Escape Sequence |
What It Represents |
|
\' |
Single quotation mark |
|
\" |
Double quotation |
|
\t |
Tab |
|
\n |
Newline |
|
\r |
Return/line feed |
|
\$ |
A literal dollar sign |
|
\\ |
Backslash |
|
\70 |
Represents the octal value |
|
\x05 |
Represents the hexadecimal character |
Example 4.5.
<html><head><title>Escape Sequences</title></head>
<body bgcolor="orange">
<b>
1 <pre>
2 <?php
3 print "\t\tTwo tabs are \\t\\t, and two newlines are
\\n\\n.\n\n";
4 print "\tThe escaped octal numbers represent ASCII
\101\102\103.\n";
print "\tThe escaped hexadecimal numbers represent ASCII
\x25\x26.\n";
5 print '\tWith single quotes, backslash sequences are not
interpreted.\n';
?>
</pre>
</b>
</body>
</html>
Explanation
|
1 |
Because this file will be displayed in a browser window, the HTML <pre> tags are used to retain spaces and tabs. If you run PHP at the command line, the escape sequences will be interpreted. |
|
2 |
The PHP program starts here with its opening tag. |
|
3 |
The escape sequences must be enclosed in double quotes. The sequences for tab (\t) and newline (\n) characters produce tabs and newlines. If a backslash is prepended with another backslash, then the backslash is treated as a literal. |
|
4 |
In this example, by preceding an octal or hexadecimal number with a backslash, its ASCII equivalent is displayed. |
|
5 |
If a string is enclosed in single quotes, escape sequences are ignored. See the output in Figure 4.6. |
)
Figure 4.6 Escape sequences and the <pre> tag.
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Figure 4.7 Escape sequences at the command line.
4.1.3 Boolean Literals
Boolean literals (introduced in PHP 4) are logical values that have only one of two values, true or false, both case insensitive. You can think of the values as yes or no, on or off, or 1 or 0. They are used to test whether a condition is true or false. When using numeric comparison and equality operators, the value true evaluates to 1 and false evaluates to the empty string (see Figure 4.8).
$answer1 = true;
or
if ($answer2 == false) { do something; }
Example 4.6.
<?php
if ( 0 == False && "" == FALSE) {
print "zero and null are <em>false.</em><br /> ";}
if ( 1 == True && "abc" == true) {
print "1 and \"abc\" are both <em>true.</em><br /> "; }
?>
)
Figure 4.8 True and false.
4.1.4 Special Data Types
Null
NULL represents "no value," meaning "nothing," not even an empty string or zero. It is a type of NULL. An uninitialized variable contains the value NULL. A variable can be assigned the value NULL, and if a variable has been unset, it is considered to be NULL.
Resource
A resource is a special variable, holding a reference to an external resource such as a database object or file handler. Resources are created and used by special functions. File and database resources are defined by the PHP interpreter and are only accessible by functions provided by the interpreter (see Chapter 11, "Files and Directories," and Chapter 15, "PHP and MySQL Integration).
The gettype() Function
The gettype() built-in function returns a string to identify the data type of its argument. The argument might be a variable, string, keyword, and so on. You can use the gettype() function to check whether or not a variable has been defined because if there is no value associated with the variable, the gettype() function returns NULL (see Figure 4.9).
)
Figure 4.9 PHP data types. Output from Example 4.7.
Strings returned from the gettype() function include the following:
- "boolean" (since PHP 4)
- "integer"
- "double" (for historical reasons "double" is returned in case of a float, and not simply "float")
- "string"
- "array"
- "object"
- "resource" (since PHP 4)
- "NULL" (since PHP 4)
Format
string gettype ( mixed var )
Examples:
$type=gettype(54.6); // Returns "float"
print gettype("yes"); // Returns and prints "string"
Example 4.7.
<html>
<head><title>Getting the Data Type with gettype()</title>
</head><body bgcolor="lightblue">
<font face = "arial" size = '+1'>
<pre>
<?php
print "Type <b>5623</b> is: " . gettype(5623) . ".\n";
print "Type <b>-22</b> is: " . gettype(-22) . ".\n";
print "Type <b>15.3</b> is: " . gettype(15.3) . ".\n";
print "Type <b>5e3</b> is: " . gettype(5e3) . ".\n";
print "Type <b>\"Hi\"</b> is: " . gettype("Hi") . ".\n";
print "Type <b>true</b> is: " . gettype(true) . ".\n";
print "Type <b>false</b> is: " . gettype(false) . ".\n";
print "Type <b>null</b> is: " . gettype(null) . ".\n";
print "Type <b>\$nothing</b> is: " . gettype($nothing) . ".\n";
?>
</body>
</html>