PHP maintains a connection status bitfield with 3 bits:
By default a PHP script is terminated when the connection to the client is broken and the ABORTED bit is turned on. This can be changed using the ignore_user_abort() function. The TIMEOUT bit is set when the script timelimit is exceed. This timelimit can be set using set_time_limit().

<?php
set_time_limit(0);
ignore_user_abort(true);
/* code which will always run to completion */
?>
You can call connection_status() to check on the status of a connection.

<?php
ignore_user_abort(true);
echo "some output";
if(connection_status()==0) {
    // Code that only runs when the connection is still alive
} else {
    // Code that only runs on an abort 
}
?>
You can also register a function which will be called at the end of the script no matter how the script was terminated.

<?php
function foo() {
    if(connection_status() & 1) 
        error_log("Connection Aborted",0);
    if(connection_status() & 2) 
        error_log("Connection Timed Out",0);
    if(!connection_status()) 
        error_log("Normal Exit",0);
}
register_shutdown_function('foo');
?>