PHP Developer’s Dictionary IT-SC book 20 This section describes PHP's use of arrays, strings, type conversions, variables, and constants. No programming language would be complete without these necessary items. If you are familiar with other programming languages, these should be very intuitive. The syntax for most of these items is similar to the C programming language and their functionality is as you would expect. The following sections are not intended to be an exhaustive review of arrays, variables, and type conversions, but rather a basic review. Arrays As with every programming language, PHP would be incomplete if it did not include the capability to define and manipulate arrays. PHP supports arrays that are indexed by number and associative arrays. PHP also supports multidimensional arrays. Arrays are variables that contain multiple elements indexed by numbers or names. This means that a variable called FirstName[1] could contain the value Kaitlin , whereas another variable called FirstName[2] could contain the value Austin . The variable name is FirstName , and the index is the [1] or [2]. The array() function in PHP is used to define an array and assign values to it. This function would be used like this: $CustomerName = array ("Jean", "Loren", "Ted", "Gladys" ); The elements in the $CustomerName array can now be accessed by using the variable name and the index in the following way: print "$CustomerName[3]"; This would print the name Gladys , which is actually the fourth item in the array. Remember that array indexes begin at 0. Associative arrays are indexed with strings instead of numbers. This is useful when describing the contents of the element using the index. To define an associative array, we use the array() function and we must define both the key and value for each element. In the following example, we create an associative array called $Contacts with three elements: FirstName , Phone , and Email. $Contacts = array(Name=>"Eric", Phone=>"289- 9272",Email=>"eric@winslow.com"); We can now access any of the fields of $Contacts like this: print $Contacts[Email]; This would print the value eric@winslow.com . PHP also supports multidimensional arrays. A multidimensional array can be thought of as an array of arrays. In other words, we could define the $Contacts array to contain multiple entries for different contacts. That definition would look like this: PHP Developer’s Dictionary IT-SC book 21 $Contacts[1] = array(Name=>"Eric",Phone=>"289- 9272",Email=>"eric@winslow.com"); $Contacts[2] = array(Name=>"Ryan",Phone=>"289- 9446",Email=>"ryan@winslow.com"); Now, if we want to retrieve Ryan's email address, the code looks like this: print $Contacts[2][Email]; PHP also has many functions that enable the programmer to manipulate, sort, and return information about the array. These functions make it easy to access your array information quickly and easily. Details about these functions are described in Chapter 5, "PHP Language Extensions." Strings PHP has many functions that enable you to manipulate and format strings. These functions provide methods of determining the length of a string, finding a substring in a string, removing white space from a string, replacing substrings, and changing the case of a string. This section does not discuss all these cases, but it does provide a general overview of these functions. If you are familiar with the C programming language, you recognize the printf() function. This function enables you to output a string and variables in a multitude of different ways. Consider the following example: printf("This is a formatted number: %d\ n", 1957 ); This prints out the number, 1957, as a decimal value, which is defined by the %d specifier. There are other specifiers available to the printf() function. These include specifiers that will print the value as a hexadecimal, ASCII, octal, binary, or floating- point number. Chapter 5 discusses this function in detail and describes all the available options. The strlen() function returns the length of the string. This function is useful for error checking and formatting. The following is an example of how to use this function and what information is returned: $Test = "This is a test string"; print strlen($Test); // prints out 21 The strstr() function returns part of the entire string. The portion of the string that is returned is defined by an integer number. This integer number defines the number of characters to return. A positive number starts counting characters from the front of the string; a negative number counts from the end of the string. An example of this function follows: $date = "03-October-1957"; $Test = substr( $date, 10 ) print $Test; // Prints out the string "03-October"; PHP Developer’s Dictionary IT-SC book 22 The trim() function is useful when a variable that is to be used has white space or padding that needs to be removed. This function removes the spaces from the beginning and the end of the string. An example of this is $Test = " A String with leading and trailing spaces "; $Test = trim($Test); print $Test; // prints out the string without the spaces The strtoupper() and the strtolower() functions convert the specified string to all uppercase or all lowercase, respectively. Often when you are doing string comparisons, it is a problem to test all possible cases. These functions convert the string to one case, and thus make it easier to evaluate comparisons. The following is an example of the strtoupper() and strtolower() functions in use: $Test = "ThIs Is A mIxEd Up StRiNg"; $Upper = strtoupper( $Test ); print $Upper; // prints out the string "THIS IS A MIXED UP STRING" $Lower = strtolower( $Test ); print $Lower; // prints out the string "this is a mixed up string" This is by no means the entire list of string functions that are available in PHP. The entire list of functions, along with their complete descriptions, is in Chapter 5 . Type Conversion In many programming languages, it is necessary to declare the variable as some type. Different types of variables are handled in different ways and take different amounts of memory. PHP does not require you to declare a variable's type when the variable is initialized. PHP calculates the variable's type based on what value the variable has. For instance, if a variable has the value of 3 , the variable is typed as integer. If the variable has a value of 3.1415927 , the variable is typed as a double. Data types in PHP can be integer, double, string, boolean, object, or array. PHP has some functions to enable you to detect what a variable's type is. The function gettype() returns a variable's type based on the value that is passed into the function. PHP also gives you the ability to set an integer's type by using the settype() function. Type casting is also supported in PHP. To cast a variable as a certain type, you place the name of the data type in parentheses in front of the variable. For example, to cast a copy of the variable named $pi to a double, the code is $pi = 3.1415927 $copyofpi = (double)$pi; print $copyofpi; This example prints out the value 3.1415927 . If the same variable were cast to an integer, the code would be $pi = 3.1415927 $copyofpi = (integer)$pi; print $copyofpi; PHP Developer’s Dictionary IT-SC book 23 The output of this example would be 3 because the variable would contain no decimal point. Variables Variables in PHP come in many forms, but they are always preceded with a dollar sign ( $ ). Variables can contain letters, numbers, and the underscore ( _ ) character. Variables must contain only alphanumeric characters and must not contain any spaces. PHP also enables you to use something known as dynamic variables . A dynamic variable is one in which the variable name can be stored in a variable. This can be quite confusing, but quite useful at the same time. Please consider the following example: $FirstName = "Kaitlin"; This variable declaration is the same as $Variable="FirstName"; $$Variable = "Kaitlin"; The $Variable variable contains the string "FirstName" . You can think of the variable $$Variable as a single $ followed by the value of $Variable or "FirstName" . Constants Constants are values that are defined in your PHP script and that do not change. Constants are defined by using the define() function. After the constant is defined, it cannot be changed. Constants are accessed by using the name of the constant only. There is no dollar sign preceding the name of the constant. For example, the constant PI is defined like this: define( "PI",3.1415927); To access the value of PI , the code would look like this: $Circumference = PI*($Radius*$Radius); Note that constants are usually defined using all capital letters. PHP also has some internal constants. For instance, the constant __FILE__ returns the name of the file that PHP is currently reading. The constant __LINE__ returns the line number of the file. These constants are normally used when you are generating error messages. Operators and Mathematical Functions PHP Developer’s Dictionary IT-SC book 24 PHP, like other programming languages, provides functions for performing many types of mathematical functions. Details of these mathematical functions are described in Chapter 5 . This section gives you an overview of the mathematical operators and functions that are available in PHP and examples of how to use them. Expressions and Operators Mathematical expressions consist of operators and operands. Usually, two operands are connected by an operator to create the expression. Consider this example: 6=2+4 The numbers 2 and 4 are operands, the operator is the plus (+) sign, and the result is 6. This entire example is referred to as an expression. PHP has four different types of operators: assignment operators, mathematical operators, comparison operators, and logical operators. Assignment operators assign a specific value to a variable. Assignment operators are described in Table 1.1 . Table 1.1. Assignment Operators Operator Example Description = $x = 2 Assign the number 2 to $x += $x += 2 Assign the value of $x + 2 to $x -= $x -= 2 Assign the value of $x - 2 to $x /= $x /= 2 Assign the value of $x divided by 2 to $x *= $x *= 2 Assign the value of $x multiplied by 2 to $x %= $x %= 2 Assign the value of $x to the modulus 2 of $x .= $x .= " value" Assign the value of $x to the value of $x concatenated with " value" The mathematical operators in PHP do exactly what you would expect. They include the basic addition, subtraction, division, and multiplication operators. PHP also uses the modulus operator. This operator divides the left operand by the right operand, and returns the remainder. Table 1.2 lists the mathematical operators available in PHP. Table 1.2. Mathematical Operators Operator Description + Addition - Subtraction / Division * Multiplication % Modulus Comparison operators compare the left and right operands and return either a Boolean true or Boolean false. The comparison operators are listed in Table 1.3 . Table 1.3. Comparison Operators Operator Description == Equivalent to != Not equivalent to === Identical to . programming language, PHP would be incomplete if it did not include the capability to define and manipulate arrays. PHP supports arrays that are indexed by number and associative arrays. PHP also supports. PHP Developer’s Dictionary IT-SC book 20 This section describes PHP& apos;s use of arrays, strings, type conversions, variables,. and easily. Details about these functions are described in Chapter 5, " ;PHP Language Extensions." Strings PHP has many functions that enable you to manipulate and format strings. These