$numchars = rand(4,15);
$chars = explode(',','a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z,A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z,0,1,2,3,4,5,6,7,8,9');
$random='';
for($i=0; $i<$numchars;$i++)
{
$random.=$chars[rand(0,count($chars)-1)];
}
$unique_id=$random;
it will generate alphnumeric random number every time when page is refreshed.