UTK CS 460 - PHP - Constructs and Variables (14 pages)

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PHP - Constructs and Variables



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PHP - Constructs and Variables

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Pages:
14
School:
The University of Tennessee, Knoxville
Course:
Cs 460 - Operating Systems
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PHP Constructs and Variables Introduction This document describes 1 the syntax and types of variables 2 PHP control structures i e conditionals and loops 3 mixed mode processing 4 how to use one script from within another 5 how to define and use functions 6 global variables in PHP 7 special cases for variable types 8 variable variables 9 global variables unique to PHP 10 constants in PHP 11 arrays indexed and associative and 12 common array number and string manipulation functions Brief overview of variables The syntax for PHP variables is similar to C and most other programming languages There are three primary differences 1 Variable names must be preceded by a dollar sign 2 Variables do not need to be declared before being used 3 Variables are dynamically typed so you do not need to specify the type e g int float etc Here are the fundamental variable types which will be covered in more detail later in this document Numeric o integer Integers 231 values outside this range are converted to floating point o float Floating point numbers o boolean true or false PHP internally resolves these to 1 one and 0 zero respectively Also as in C 0 zero is false and anything else is true string String of characters array An array of values possibly other arrays Arrays can be indexed or associative i e a hash map object Similar to a class in C or Java NOTE Object oriented PHP programming will not be covered in this course resource A handle to something that is not PHP data e g image data database query result PHP has a useful function named var dump that prints the current type and value for one or more variables Arrays and objects are printed recursively with their values indented to show structure a 35 b Programming is fun c array 1 1 2 3 var dump a b c Here s the output from the above code int 35 string 19 Programming is fun array 6 0 int 1 1 int 1 2 int 2 3 int 3 The variable a is an integer with value 35 The variable b is a string that contains 19 characters The variable c is an array with six elements element zero is an integer whose value is 1 and so on Control structures The control structures conditionals and loops for PHP are nearly identical to C The following list identifies how PHP s control structure syntax differs from other languages The else if condition is denoted by elseif Recall that else if is used in C and elsif for Perl Single statements within a condition or loop do not require curly braces unlike Perl where the braces are mandatory The cases within a switch statement can be strings unlike C where the cases must be numeric The syntax for the foreach loop is slightly different than Perl For example in Perl you would write foreach val array In PHP you would write foreach array as val Mixed mode processing When the PHP interpreter encounters code islands it switches into parsing mode This feature is significant for two reasons you can retain variable scope and you can distinguish PHP code from markup Here are two examples that demonstrate these concepts php username dknuth h1 Hello World h1 p Welcome php print username Enjoy your stay p Defining a variable in the first code island Using a variable defined in a previous code island Even though there is HTML markup between the two code islands the variable username retains its value The technical reason for this capability is that the variable username is within the current file s scope The following example demonstrates how to have specific HTML markup displayed if a given condition is true php if is logged in true p Welcome member a href logout php Log out a p p Check out our new member features below p php else p a href register php Register for an account a p p You must be a member to view anything on this site sorry p php The same result could be achieved by using multiple print statements within the condition blocks to output the HTML markup Including other scripts As with most programming languages initialization function definitions and common code can be placed in separate files For example if you had several constants used by multiple C applications those constants would be defined in a common header file rather than being duplicated within each source code file In the case of PHP these separate scripts typically contain common shared functions object definitions and page layout code To include other scripts use the include statement include somefile php The PHP interpreter essentially inserts the contents of the specified file name into the current location If you try to include a file that does not exist a warning message will be displayed in the browser NOTE If PHP is configured so that display errors is set to Off the warning will not be seen in the browser You can however see the warning by running the script using the CLI Suppose you want to define the heading for each page on your site in the file pageheader php php print div class pageheader Programming is Fun div n print div class commonlinks some links would go here div n print p Motto i If programming isn t fun you re doing it wrong i p n This script can be included by other PHP files For example the index page index php may look like this DOCTYPE html PUBLIC W3C DTD XHTML 1 0 Transitional EN http www w3 org TR xhtml1 DTD xhtml1 transitional dtd html xmlns http www w3 org 1999 xhtml head meta http equiv content type content text html charset utf 8 title Programming is Fun title head body php include pageheader php More code here body html Potentially each page of your site would use the page header so that a you don t have to copy and paste the same header markup into every PHP file that displays HTML content and b if you decide to change the page header content you only need to modify pageheader php The inclusion of other scripts can also be based on a condition Suppose you want to display additional content if the current user is an administrator NOTE This is a contrived example the variable is admin has no special meaning php if is admin true include admincontent php If there are external scripts that contain code that is mandatory for the current script use the require statement require somefile php If a required file does not exist an error message will be displayed in the browser and the PHP interpreter will exit i e a fatal error NOTE If PHP is configured so that display errors is set to Off the error message will not be seen in the browser You can however see the error by running the script using the CLI If you want to prevent multiple includes


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