$previewcontent = substr($content,0,300);

if(isset($go)) { //If set then they have seen the preview already
echo $content; //Display your usual content
} else { //First time viewing this page
echo "Article preview:<br><br>" . $previewcontent;
echo "See this entire post<a href=\"showcontent.php?id=" . $id . "&go=yes\">here</a>";

//Or you could add something like this (instead of lines 6 & 7) for paid/subscription content

if(isset($user)) { //If a user is logged in...
echo "Go ahead and <a href=\"showcontent.php?id=" . $id . "&go=yes\">view</a>";
} else { //$user is not set - insert login/signup link/form below
echo "To view this solution please click <a href=\"yoursignupscript.php\">here</a> to purchase access.";
}