Implementing Timer in PHP and Sleep function

I am creating an PHP application in which i want to set a timer of 15 minutes for a particular section of test. If the student doesn't complete the section in 15 minutes the scores are saved and the student is navigated to the next section.

So i want to create a timer and want to display the current time left. But i don't know how to implement it and also if we use sleep() function that will it interrupt the working of other functions as well.

jQuery will help you on this...

Use below function for timer

var hours1=0, minutes1=0, seconds1=0;

function calculate() {

setTimeout(calculate, 1000);
if((hours==0)&&(minutes==0)&&(seconds==0)){$('#submit-btn').trigger('click');} // HERE YOU CAN SWITCH TO NEXT QUESTION
h1 = makeTwoDigit(hours);   m1 = makeTwoDigit(minutes); s1 = makeTwoDigit(seconds);
h2 = makeTwoDigit(hours1);  m2 = makeTwoDigit(minutes1);    s2 = makeTwoDigit(seconds1);
$('title').html(h1+':'+m1+':'+s1);
$('#time-taken').val(h2+':'+m2+':'+s2);
$('#minutes').html(m1);
$('#seconds').html(s1);

seconds--; 
if (seconds < 0) { 
    seconds = 59; minutes--;

    if (minutes < 0) {
        hours--;
        minutes = 59;                        
    }
}
seconds1++; 
if (seconds1 > 59) { 
    seconds1 = 00; minutes1++;      
    if (minutes1 >59) {
        hours1++;
        minutes1 = 00;                        
    }
}

}

function makeTwoDigit(num) {
    if (num < 10) { return "0" + num;} return num;
}

Thanks