The number of rows returned by fbsql_num_rows is wrong when used with 'select DISTINCT' query. This script shows differences between fbsql_num_rows function and a while over all rows. This problem doesn't appear if you don't use the 'distinct' option.
Greetings
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$conn=fbsql_connect("127.0.0.1","myname","mypassword");
fbsql_select_db("mybase",$conn)	;
$sql = "select distinct id_actor from actor_table;";
$result=fbsql_query($sql,$conn) ;

$num_rows = fbsql_num_rows($result); 
echo "$num_rows Rows\n";// displays "2147483647 Rows"

$i=0;
while ($row=fbsql_fetch_array($result)) 
	$i++;
echo "$i Rows\n";	// displays "5 Rows"

@fbsql_free_result($result);
@fbsql_close($conn);