"Script triggers do not work in Instant Web Publishing (IWP) and Custom Web Publishing (CWP) when events are triggered by user interaction. However, if a script (non-user interaction) is executed and it would cause a script trigger to execute a script, then the triggered script will execute as expected. For example, if an IWP user tabs into a field that has an OnObjectEnter script trigger attached to it, the trigger will not execute the associated script. However, if a script step causes focus to move into that field then the OnObjectEnter script trigger attached to the field will execute the script as expected."
As I read this, you cannot trigger a script directly from a php-page. Maybe the way to do it is to have scripts running on the server ( if you have one ) and monitor the field content/changes from there.
I guess in this case I will fire a script out of the php page which should do the same as the trigger.