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Sample Chapter: PHP Pocket Reference


Never worked with PHP? Curious? In this article Tim takes a look at a sample chapter from O'Reillys PHP Pocket Reference. This chapter is a great introduction to PHP and is explained simply and clearly.

Author Info:
By: Tim Pabst
Rating: 5 stars5 stars5 stars5 stars5 stars / 7
June 15, 2002
TABLE OF CONTENTS:
  1. · Sample Chapter: PHP Pocket Reference
  2. · Introduction
  3. · Installation and Configuration
  4. · Including Files
  5. · Arrays
  6. · Control Structures
  7. · Functions
  8. · Showing the Browser and IP Address
  9. · Web Database Integration
  10. · Conclusion

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Sample Chapter: PHP Pocket Reference - Functions
(Page 7 of 10 )

A function is a named sequence of code statements that can optionally accept parameters and return a value. A function call is an expression that has a value; its value is the returned value from the function. PHP provides a large number of internal functions. The "Function Reference" section lists all of the commonly available functions. PHP also supports user-definable functions. To define a function, use the function keyword. For example:

function soundcheck($a, $b, $c) {
return "Testing, $a, $b, $c";
}


When you define a function, you need to be careful what name you give it. In particular, you need to make sure that the name does not conflict with any of the internal PHP functions. If you do use a function name that conflicts with an internal function, you get the following error:

Fatal error: Can't redeclare already declared function in filename on line N

After you define a function, you call it by passing in the appropriate arguments. For example:

echo soundcheck(4, 5, 6);

You can also create functions with optional parameters. To do so, you set a default value for each optional parameter in the definition, using C++ style. For example, here's how to make all the parameters to the soundcheck( ) function optional:

function soundcheck($a=1, $b=2, $c=3) {
return "Testing, $a, $b, $c";
}


Variable Scope
The scope of a variable refers to where in a program the variable is available. If a variable is defined in the main part of a PHP script (i.e., not inside a function or a class), it is in the global scope. Note that global variables are only available during the current request. The only way to make variables in one page available to subsequent requests to another page is to pass them to that page via cookies, GET method data, or PUT method data. To access a global variable from inside a function, you need to use the global keyword. For example:

function test( ) {
global $var;
echo $var;
}
$var="Hello World";
test( );


The $GLOBALS array is an alternative mechanism for accessing variables in the global scope. This is an associative array of all the variables currently defined in the global scope:

function test( ) {
echo $GLOBALS["var"];
}
$var="Hello World";
test( );


Every function has its own scope. When you create a variable inside of a function, that variable has local scope. In other words, it is only available within the function. In addition, if there is a global variable with the same name as a variable within a function, any changes to the function variable do not affect the value of the global variable.

When you call a function, the arguments you pass to the function (if any) are defined as variables within the function, using the parameter names as variable names. Just as with variables created within a function, these passed arguments are only available within the scope of the function.

Passing Arguments
There are two ways you can pass arguments to a function: by value and by reference. To pass an argument by value, you pass in any valid expression. That expression is evaluated and the value is assigned to the corresponding parameter defined within the function. Any changes you make to the parameter within the function have no effect on the argument passed to the function. For example:

function triple($x) {
$x=$x*3;
return $x;
}
$var=10;
$triplevar=triple($var);


In this case, $var evaluates to 10 when triple( ) is called, so $x is set to 10 inside the function. When $x is tripled, that change does not affect the value of $var outside the function.

In contrast, when you pass an argument by reference, changes to the parameter within the function do affect the value of the argument outside the scope of the function. That's because when you pass an argument by reference, you must pass a variable to the function. Now the parameter in the function refers directly to the value of the variable, meaning that any changes within the function are also visible outside the function. For example:

function triple($x) {
$x=$x*3;
return $x;
}
$var=10;
triple(&$var);


The & that precedes $var in the call to triple( ) causes the argument to be passed by reference, so the end result is that $var ends up with a value of 30.

Static Variables
PHP supports declaring local function variables as static. A static variable retains its value between function calls, but is still accessible only from within the function it is declared in. Static variables can be initialized and this initialization only takes place the first time the static declaration is executed. Static variables are often used as counters, as in this example:

function hitcount( )
static $count = 0;

if ($count == 0) {
print "This is the first time this page";
print " has been accessed";
}
else {
print "This page has been accessed $count";
print " times";
}
$count++;
}


Web-Related Variables
PHP automatically creates global variables for all the data it receives in an HTTP request. This can include GET data, POST data, cookie data, and environment variables. Say you have an HTML form that looks as follows:

<FORM ACTION="test.php3" METHOD="POST">
<INPUT TYPE=text NAME=var>
</FORM>


When the form is submitted to the test.php3 file, the $var variable within that file is set to whatever the user entered in the text field.

A variable can also be set in a URL like this:

http://your.server/test.php3?var=Hello+World

When the request for this URL is processed, the $var variable is set for the test.php3 page.

Any environment variables present in your web server's configuration are also made available, along with any CGI-style variables your web server might set. The actual set of variables varies between different web servers. The best way to get a list of these variables is to use PHP's special information tag. Put the following code in a page and load the page in your browser:

<? phpinfo( ) ?>

You should see a page with quite a bit of information about PHP and the machine it is running on. There is a table that describes each of the extensions currently enabled in PHP. Another table shows the current values of all the various configuration directives from your php3.ini file. Following those two tables are more tables showing the regular environment variables, the special PHP internal variables, and the special environment variables that your web server has added. Finally, the HTTP request and response headers for the current request are shown.

Sometimes it is convenient to create a generic form handler, where you don't necessarily know all the form element names. To support this, PHP provides GET, POST, and cookie associative arrays that contain all of the data passed to the page using the different techniques. These arrays are named $HTTP_GET_DATA, $HTTP_POST_DATA, $HTTP_COOKIE_DATA, respectively. For example, here's another way to access the value of the text field in our form:

echo $HTTP_POST_VARS["var"];

PHP sets global variables in a particular order. By default, global variables are set first from GET data, then from POST data, and then finally from cookie data. This means that if you have a form with a field named var that uses the GET method and a cookie with a var value, there is just one global variable named $var that has the value of the cookie data. Of course, you can still get at the GET data through the $HTTP_GET_DATA array. The default order can be defined with the gpc_order directive in the php3.ini file.

Examples
The best way to understand the power of PHP is to examine some real examples of PHP in action, so we'll look at some common uses of PHP in this section.
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