php preg_replace function

[eluser]basty_dread[/eluser]
Hello,
I need your help guys about this preg replace function

this is the string i need to replace:

Code:

$string = "the quick brown fox jumps over the lazy dog, Brown Fox";
$find = "brown fox";

the output should be

the quick brownOfox jumps over the lazy dog, BrownOFox

can you help me with the right preg replace.. please. Thank you..

Code:

$save = preg_replace('/\b('.$find.')\b/i', <i dont know what to put here>, $string );

many thanks..