The problem is multiline strings in javascript need to be split with a \
at the end of each line.
function alert($msg) {
echo '<script type="text/javascript">alert("'.$msg.'")</script>';
}
$msg = 'testtesttesttesttesttesttesttesttesttesttesttesttesttesttesttest.\
testtesttesttesttesttesttesttest.\
testtesttesttesttesttesttesttesttesttest.';
alert($msg);