Create sub domain application with PHP

First you need to set your main domain to act as a catch-all subdomain. its like putting ServerAlias * so that each subdomain which is requested
to the main domain reaches the same place. I mean if your main domain root is

/home/admin/abc.com/htdocs/

then your catch-all setup shall send all sub-domain requests to that same directory

/home/admin/abc.com/htdocs/

and then in the index.php file of this directory you can do this

global $domainname;
global $subdomainname;
$domainarray = explode('.', $_SERVER['HTTP_HOST']);
$index=count($domainarray)-1;
$domainname= $domainarray[$index-1].".".$domainarray[$index];
$subdomainname="";
for($i=0;$i<$index-1;$i++)
{
if($subdomainname=="")
{
$subdomainname=$domainarray[$i];
}
else
{
$subdomainname=$subdomainname.".".$domainarray[$i];
}

}

ShowCustomizedPageForsubdomain($subdomainname);

?>

This function ShowCustomizedPageForsubdomain($subdomainname) can be easily implemented in two ways:

1) Either your store Page's Html in your database and all this function would do is to pick up that html code for the provided sub-domain from the database and simply
"echo" or "print" it.
2) or you can simply have standard actual pages for each subdomain and you simply include them here like this

ob_start();
include "http://url_of_the_page_you_want_to_show";
$data=ob_get_contents();
ob_clean();

this Object buffer will return you that page's content in the variable "$data"; you simply echo $data;

Second Implementation of your function is like this

function ShowCustomizedPageForsubdomain($subdomainname)
{
ob_start();
include "http://url_of_the_page_you_want_to_show_for_this_subdomain";
$data=ob_get_contents();
ob_clean();
echo $data;
}