Whether a file named BBB.php is there or not there

I have a file named “AAA.php”.
I like to know whether there is BBB.php or there is no BBB.php in the directory where AAA.php is.

[b]would-be code in AAA.php[/b]

if(exist(BBB.php) ) {
echo "Yes, there is BBB.php.";
echo "No, BBB.php doesn't exist in the directory.";