Pass a PHP string to a JavaScript variable (and escape newlines) [duplicate]

What is the easiest way to encode a PHP string for output to a JavaScript variable?

I have a PHP string which includes quotes and newlines. I need the contents of this string to be put into a JavaScript variable.

Normally, I would just construct my JavaScript in a PHP file, à la:

<script>
  var myvar = "<?php echo $myVarValue;?>";
</script>

However, this doesn’t work when $myVarValue contains quotes or newlines.