var pop //create global variable without assigning a value so that its value is null

function popUp(URL,w,h){

if(pop!=null && !pop.closed){ // if popUp has been opened, and popUp hasn't already been closed
pop.close() //close it in order to re-assign properties
}

pop = window.open(URL,"win1","left=350 top=320 width="+w+",height="+h+",resizable=0,scrollbars=0")

pop.focus() ;
} 
/*onClick="popUp('projekti_vise.php?projekt=0100010', '400', '160'); // primjer otvaranja prozora*/