Store php code in a variable to write to file

Hi Experts,

I am trying to write an automated setup script so that when user types in details on a form a config file is written to the server.

This is my code:

$handle = fopen($setting['config']['folder'] . $setting['config']['file'], 'w');
	

$input = "<?php
// Generated ".date('F j, Y H:i:s')."

// Database Details

$config['hostname'] = '$db_host';
$config['username'] = '$db_user';
$config['password'] = '$db_pass';
$config['database'] = '$db_name';
$config['table_prefix'] = 'sadmin_';
$config['universal_admin_url'] = '$admin_url';
$config['base_url'] = '$base_ur';
$config['seo_page'] = '$seo_page';
$config['show_errors'] = true;
$config['webmaster_email'] = '$webmaster_email';
$config['webmaster_name'] = '$webmaster_name';
$config['language'] = 'english';
?>
";

fwrite($handle, $input);
fclose($handle);
if (file_exists($setting['config']['folder'] . $setting['config']['file']))
	echo '<h3>Configuration file created!</h3><p>The file <code>'.$setting['config']['folder'] . $setting['config']['file'].'</code> has been created successfully. To continue the installation, please continue the installation. </p><a href="?step=3" class="button" style="float:right;"> <span>Generate Database Tables &rsaquo;</span> </a> ';
else
	echo '<h3>ERROR!</h3><p>Configuration file was not created. Please check the the folder <code>'.$setting['config']['folder'].'</code> is created and the permissions to the  folder are set to <code>777</code>. After checking, click the link button below to regenerate the configuration file again..</p> <a href="?step=1" class="button fail" style="float:left"> <span>&lsaquo; Regenerate Configuration</span> </a>';

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But its not working. What I am trying to achieve is the config file shows this inside

<?php

// Generated September 6, 2011 16:00:00

// Database Details

$config['hostname'] = "localhost";
$config['username'] = "jim";
$config['password'] = "password";
$config['database'] = "testdb";
$config['table_prefix'] = "td_";
$config['universal_admin_url'] = "http://www.jimclarke.co.uk/admin";
$config['base_url'] = "http://www.jimclarke.co.uk/";
$config['seo_page'] = "index2.php";
$config['show_errors'] = true;
$config['webmaster_email'] = 'me@example.com';
$config['webmaster_name'] = 'Administrator';
$config['language'] = 'english';
?>

Where am I going wrong?

Thanks
Jim