needle: /       
string: o/a/i

if before_needle is 1  then needle is excluded: "o/a"
if before_needle is false  then needle is included:  "/i" 

as code:

$str="o/a/i";
$cs="/";

echo "if before_needle == 1  then needle is excluded"."\n";
$str="o/a/i";
$cs="/";
echo mb_strrchr ($str,$cs,1)."\n";

echo "if before_needle == false  then needle is included"."\n";
$str="o/a/i";
$cs="/";
echo mb_strrchr ($str,$cs,0)."\n";