ITEC 325 2011spring ibarland

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ch10-funcs
functions
chapter 10

From PHP Visual Quickstart Guide by Larry Ullman
Based on notes by Jack Davis (jcdavis@radford.edu)

Creating functions can save you oodles of time over the course of your programming projects. They are needed if you want to create well structured Web applications. You should use functions to modularize code and to build reusable code. You are encouraged to make a file for the reusable parts of data verification, as well as a file of for common utility functions, and include these functions from other files.

• To create a function:
  

              function whatever()
              {
                print 'whatever';
              }
              

• To call the function:
  whatever();
• Any valid PHP code can go within the statement(s) area of the function, including calls to other functions. There is also no limit on the number of statements a function contains; but make sure each statement ends with a semicolon just as you would within the rest of the PHP script.
• Here's an example function that generates a pull down menu.
  ch10-funcs-make-pulldown.php
• the (meta)function function_exists() function takes in a string, and returns whether or not a function by that name exists in the PHP program.
• formal parameters are defined in the function header. HOWEVER no checking is done for the number or type of variables passed to the function!
  function whatever($a, $b)
• Parameters can be given default values, if ... function whatever($a=0,$b="abc") { … }
• Functions can return values
  

               function whatever()
               {
                 print ("Hello");
                 return "abc";
               }
               

• To return multiple values, a function can return an array
• Variable Scope
  

  Variables defined within a function exist only within the function, as you'd expect.
  Non-standard: However, variables declared outside the function -- at the file's top-level -- are not in scope in your function!, unless you explicitly "import" them with the keyword global:

  function whatever() { global $a; // We are using $a taken from the enclosing context. $a = $a+1; … }

  This is an easy bug to miss, since php will invent a new local variable with a zero-ish value, if you forget to use global.

  Minimize the use of global variables.
  Consider passing in information the function needs as a parameter, rather than use a global variable. (That's the whole reason parameters were invented!)

  The reason for this non-standard scoping behavior? Here's my guess: in a web-scripting language, it's common to scrape code from other people's pages w/o fully understanding how it works. If you see the keyword global, you (and the compiler) are cued in to the fact that you need to copy (perhaps much) more code in order for the function to work correctly.

• Arrays are also passed by value in PHP (by default).
• You can explicitly pass a parameter by reference by using the ‘&’ character:
  

  function swap(&$a, &$b) { // Any changes made to $a will be // reflected back in the calling program $tmp = $a; $a = $b; $b = $tmp; }

  When possible, pass variables by value; this keeps any bugs being local bugs. Use return to pass information back to the caller. Also, if you pass variables by reference, the user of course has to provide variables (not any expression): neither argument swap(17,$x+5) makes sense as a reference. One reason to pass-by-reference is when you are passing large arrays that you don't want to be put on the stack.
• Note that named constants, unlike variables, are always global. So are the variables PHP calls “superglobal”: $_POST, $_REQUEST, $_COOKIE, ….

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©2011, Ian Barland, Radford University Last modified 2011.Mar.13 (Sun)

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