PHP and MySQL Web Development - P77 pptx

352 Chapter 16 Interacting with the File System and the ServerUsing Program Execution Functions We’ll move away from the file system functions now, and look at the functions that are ava

352 Chapter 16 Interacting with the File System and the Server

Using Program Execution Functions

We’ll move away from the file system functions now, and look at the functions that are available for running commands on the server

This is useful when you want to provide a Web-based front end to an existing com-mand line-based system For example, we have used these comcom-mands to set up a front end for the mailing list manager ezmlm.We will use these again when we come to the case studies later in this book

There are four main techniques you can use to execute a command on the Web serv-er.They are all pretty similar, but there are some minor differences

1 exec() The exec()function has the following prototype:

string exec (string command [, array result [, int return_value]])

You pass in the command that you would like executed, for example, exec("ls -la");

The exec()function has no direct output

It returns the last line of the result of the command

If you pass in a variable as result, you will get back an array of strings repre-senting each line of the output If you pass in a variable as return_value, you

will get the return code

2 passthru() The passthru()function has the following prototype:

void passthru (string command [, int return_value])

The passthru()function directly echoes its output through to the browser (This

is useful if the output is binary, for example, some kind of image data.)

It returns nothing

The parameters work the same way as exec()’s parameters do

3 system() The system()function has the following prototype:

string system (string command [, int return_value])

The function echoes the output of the command to the browser It tries to flush the output after each line (assuming you are running PHP as a server module), which distinguishes it from passthru()

It returns the last line of the output (upon success) or false(upon failure) The parameters work the same way as in the other functions

353 Using Program Execution Functions

4 Backticks

We mentioned these briefly in Chapter 1, “PHP Crash Course.”These are actually

an execution operator

They have no direct output.The result of executing the command is returned as a string, which can then be echoed or whatever you like

If you have more complicated needs, you can also use popen(),proc_open(), and proc_close(), which are used to fork external processes and pipe data to and from them The last two of these functions were added at PHP 4.3

The script shown in Listing 16.5 illustrates how to use each of the four techniques in

an equivalent fashion

Listing 16.5 progex.php—File Status Functions and Their Results

<?php

chdir('/uploads/');

///// exec version echo '<pre>';

// unix exec('ls -la', $result);

// windows // exec('dir', $result);

foreach ($result as $line) echo "$line\n";

echo '</pre>';

echo '<br /><hr /><br />';

///// passthru version echo '<pre>';

// unix passthru('ls -la');

// windows // passthru('dir');

echo '</pre>';

echo '<br /><hr /><br />';

///// system version

354 Chapter 16 Interacting with the File System and the Server

// unix

$result = system('ls -la');

// windows // $result = system('dir');

echo '</pre>';

echo '<br /><hr /><br />';

/////backticks version

echo '<pre>';

// unix

$result = `ls -al`;

// windows // $result = `dir`;

echo $result;

echo '</pre>';

?>

We could have used one of these approaches as an alternative to the directory-browsing script we wrote earlier Note that one of the side effects of using external functions is amply demonstrated by this code—your code is no longer portable.We have used Unix commands here and the code will clearly not run on Windows

If you plan to include user-submitted data as part of the command you’re going to execute, you should always run it through the escapeshellcmd()function first.This stops users from maliciously (or otherwise) executing commands on your system.You can call it like this, for example,

system(escapeshellcmd($command));

You should also use the escapeshellarg()function to escape any arguments you plan

to pass to your shell command

Interacting with the Environment: getenv() and putenv()

Before we leave this section, we’ll look at how you can use environment variables from within PHP.There are two functions for this purpose:getenv(), which enables you to retrieve environment variables, and putenv(), which enables you to set environment variables

Note that the environment we are talking about here is the environment in which PHP runs on the server

Listing 16.5 Continued

355 Next

You can get a list of all PHP’s environment variables by running phpinfo() Some are more useful than others, for example,

getenv("HTTP_REFERER");

will return the URL of the page from which the user came to the current page

You can also set environment variables as required with putenv(),for example,

$home = "/home/nobody";

putenv (" HOME=$home ");

If you are a system administrator and would like to limit which environment variables programmers can set, you can use the safe_mode_allowed_env_vars directive in php.ini When PHP runs in safe mode, users will only be able to set environment vari-ables whose prefixes are listed in this directive

If you would like more information about what some of the environment variables represent, you can look at the CGI specification:

http://hoohoo.ncsa.uiuc.edu/cgi/env.html

Further Reading

Most of the file system functions in PHP map to underlying operating system func-tions—try reading the man pages if you’re using UNIX for more information

Next

In Chapter 17, “Using Network and Protocol Functions,” we’ll use PHP’s network and protocol functions to interact with systems other than our own Web server.This again expands the horizons of what we can do with our scripts