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In July, 2002, I moved the leosolutions.com web site from a Microsoft
IIS web server to a Linux server running Apache to reduce the monthly
cost of web hosting. After many years of developing in Microsoft
Active Server Pages, the conversion from ASP to PHP was surprisingly
easy - nearly every statment in ASP has a one-to-one
correspondence with its PHP equivalent. However, I encountered one major
problem during the conversion process: PHP has no couterpart for application
variables in ASP.
Application variables in ASP work very similar to the
$_SESSION variable in PHP. However, unlike the
$_SESSION variable, application variables are not
specific to an individual user; they persist across every user of every page on
the web site.
The immediate benefits of application variables may not be apparent, but
esperienced ASP coders understand their power. Take this one-liner, for
example:
You are visitor number
<?php echo $_APP["visitor_count"]++; ?>
to this web site.
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Application variables have many other purposes. On the leosolutions.com
web site, I used them primarily to cache common dynamically-generated parts of
the page, such as the menu bars and project tab. (Yes, the menu bars on my
are dynamically generated from a database) Since this content doesn't
change often, it would be a waste of resources (and cause slower response times)
to generate these elements on-the-fly, each time a page was requested. Instead,
the PHP script checks the application variables for the element. If it is
found, the PHP script simply sends the HTML code stored in the application
variable directly to the output. Otherwise, the content is generated from the
database, and stored to the application variables for future use.
I looked on the web and read numerous other work-arounds for application
variables in PHP, but found none that I liked. Every one required too much
overhead code, or else multiple lines of code to actually use the variables.
Here is my solution:
app.php
<?php
define("APP_DATA_FILE",
"/tmp/application.data");
function application_start ()
{
global $_APP;
// if data file exists, load application
// variables
if (file_exists(APP_DATA_FILE))
{
// read data file
$file = fopen(APP_DATA_FILE, "r");
if ($file)
{
$data = fread($file,
filesize(APP_DATA_FILE));
fclose($file);
}
// build application variables from
// data file
$_APP = unserialize($data);
}
}
function application_end ()
{
global $_APP;
// write application data to file
$data = serialize($_APP);
$file = fopen(APP_DATA_FILE, "w");
if ($file)
{
fwrite($file, $data);
fclose($file);
}
}
?>
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The usage of the $_APP variable is very similar to
$_SESSION. Before using it on a page, you
must include app.php and call
application_start(). When you are
finished, you must call application_end().
Sample PHP file using application variables:
<?php
include("app.php");
application_start();
?>
<html>
<body>
You are visitor number
<?php echo $_APP["visitor_count"]++; ?>
to this web site.
</body>
</html>
<?php
application_end();
?>
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Numerous improvements can be made to this code, but they are not included here
for simplicity.
- For added security, permissions on the application data file should not
allow any user, except for the web server, to read or write to it.
Otherwise, if application variables contained sensitive data,
such as passwords or credit card information, other users on the system could
read this data.
- To keep from writing to disk unnecessarily, only call
application_end() if data in the
$_APP variable has changed.
- Another alternative is to let the
application_end()
function determine whether the $_APP variable has been
changed, and only write it to disk if necessary. Simply make a copy of the
$_APP variable in
application_start(), then compare
$_APP to the original in
application_end().
- Finally, don't forget that unlike $_SESSION, the
$_APP variable is only available in the global scope.
When using $_APP from inside a function, be sure to
include the statement global $_APP; at the top of the
function.
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