Find/Replace htmlentities using the standard linux toolchain?

Is there a way I can do something like the following using the standard linux toolchain?

Let's say the source at example.com/index.php is:

Hello, & world! "

How can I do something like this...

curl -s http://example.com/index.php | htmlentities

...that would print the following:

Hello, & world! "

Using only the standard linux toolchain?