Data Types and Variables

This chapter describes:

• What are PHP data types.
• How to define variables, references, and variable variable names.
• How to define and use constants.

Data Types

PHP supports 8 data types:

• boolean: It has only two literal values: 'true' and 'false'.
• integer: It is stored as signed integers with 32 bits. Integer literals have 3 forms: decimal, octal (prefixed with '0'), and hexadecimal (prefixed with '0x').
• float: It is stored as IEEE double floating point numbers with 64 bits. Float literals have the standard floating point number forms.
• string: It is a sequence of 8-bit characters. String literals have 3 forms: single quoted, double quoted, and heredoc syntax. These forms are very similar to Perl.
• array: It stores an ordered map of pairs of keys and values. This is quite different than the array type used in other languages.
• object: It stores an instance of a class.
• resource: It represents an external resource like a file, or a database connection.
• null: It represents a status of a variable in which no value is assigned the variable. Null type has only one literal value: 'null'.

Data Literals ....

PHP supports 8 data types:

• boolean: It has only two literal values: 'true' and 'false'.
• integer: It is stored as signed integers with 32 bits. Integer literals have 3 forms: decimal, octal (prefixed with '0'), and hexadecimal (prefixed with '0x').
• float: It is stored as IEEE double floating point numbers with 64 bits. Float literals have the standard floating point number forms.
• string: It is a sequence of 8-bit characters. String literals have 3 forms: single quoted, double quoted, and heredoc syntax. These forms are very similar to Perl.
• array: It stores an ordered map of pairs of keys and values. This is quite different than the array type used in other languages.
• object: It stores an instance of a class.
• resource: It represents an external resource like a file, or a database connection.
• null: It represents a status of a variable in which no value is assigned the variable. Null type has only one literal value: 'null'.

Variables

Like Perl, a PHP variable name must be prefixed with a "$" sign.

PHP variables do not need declaration. Variable can be assigned with one type of data, then re-assigned with another type of data.

Reference: PHP supports a reference concept, where a reference of a variable can be expressed with the reference operator '&', and assigned to a new variable. Once assigned with a reference, the new variable and the old variable become aliases to each other. They are sharing the storage location in memory.

Variable Variable Name: PHP supports a dynamic variable name concept, where a variable name can be expressed as a string expression in the form of ${string_expression}. If the string expression is only a single variable, it can also be written as $$variable.

To show you some of the variable features, I wrote the following PHP script, VariableTest.php:

<?php # VariableTest.php
# Copyright (c) 2002 by Dr. Herong Yang, http://www.herongyang.com/
# 
   print "\nSimple assignments:\n";
   $a = 777; # Assigning an integer
   print "   a = $a\n";
   $b = 3.14; # Assigning a float
   print "   b = $b\n";
   $c = "Hello"; # Assigning a string
   print "   c = $c\n";
   $d = true;
   print "   d = $d\n";
#
   print "\nSpecial assignments:\n";
   $a = "Test"; # Re-assigning a string
   print "   a = $a\n";
   $a = null; # Assigning no-value
   print "   a = $a\n";
   $x = &$a; # Assigning a reference
   $x = 888; # Assigning an integer to an alias. 
   print "   a = $a\n";
#
   print "\nVariable variable names:\n";
   ${'a'} = 999; # Variable variable name as a string literal
   print "   a = $a\n";
   $y = 'a';
   ${$y} = 888; # Variable variable name as a string variable
   print "   a = $a\n";
   $$y = 777; 
   print "   a = $a\n";
?>

Here is the output:

Simple assignments:
   a = 777
   b = 3.14
   c = Hello
   d = 1

Special assignments:
   a = Test
   a =
   a = 888

Variable variable names:
   a = 999
   a = 888
   a = 777

As you can see from the output, the print statement seems to behave differently than other languages:

• The print statement prints boolean true as 1.
• The print statement prints null as nothing.

Constants

PHP supports constants with the following rules:

• Constants can be defined with the define("constant_name", value) function. Constant names can not be redefined.
• Constant values can be referred by constant names.