If inputs exist, dont insert, else insert PHP mysql -

i've started php, , wondered if can help. have this

$sql="insert $tbl_name set date='$mydate' , event='$myevent'"; $result=mysql_query($sql); 

i need know how make see if event exists, , if need nothing, if doesn't insert it!

split 2 queries:

1) check if event exists. if yes nothing, else insert new event

2) continue query. way event allays exist when inserting data