You can add a cuepoint like this:

var flowPlayer = document.getElementById("myPlayer");
flowPlayer.addCuePoints([ {time: 2, name: "cue1", parameters: { foo: 1, bar: "foobar" }}, { time: 5, name: "cue2", parameters: { foo: 2, bar: "foobar2" }} ]);
}

The first cuepoint is triggered at 2 seconds in the video's timeline and the second one at 5 seconds. To make it trigger at 30% you need to calculate what time value 30% corresponds from the video's total duration.

The cuepoint callback looks like this:

function onCuePoint(cuePoint) {
alert("cue point received, time: " + cuePoint.time + ", type '" + cuePoint.type + "', name: '" + cuePoint.name + "'" + ", parameters: " + cuePoint.parameters);
}

The player will call this callback function 2 times when it reaches the 2 cuepoints. You can do the PHP server script requesting from inside this function.