How to get html data-attribute string from a php variable

I am constructing an html form in php with javascript and css, the following line of code works fine and displays a textarea with the required prefix:

$form .= '<textarea class="text" data-prefix="Message from: " ></textarea>';

However I want to include a php variable i.e

$form .= '<textarea class="text" data-prefix="Message from: $foo" ></textarea>';

This show the textarea with the prefix ‘Message from $foo’.
How do I show it with the value of $foo combined into the data-prefix i.e ‘Message from Foo’?

Any advice would be appreciated, thanks.