Remove new lines from string and replace with one empty space

$string = "
put returns between paragraphs

for linebreak add 2 spaces at end

";

Want to remove all new lines from string.

I’ve this regex, it can catch all of them, the problem is I don’t know with which function should I use it.

/\r\n|\r|\n/

$string should become:

$string = "put returns between paragraphs for linebreak add 2 spaces at end ";