How do I escape only single quotes?

Quite simply: echo str_replace('\'', '\\\'', $myString); However, I'd suggest use of JSON and json_encode() function as it will be more reliable (quotes new lines for instance):

<?php $data = array('myString' => '...'); ?>

<script>
   var phpData = <?php echo json_encode($data) ?>;
   alert(phpData.myString);
</script>

If you want to escape characters with a \, you have addcslashes(). For example, if you want to escape only single quotes like the question, you can do:

echo addcslashes($value, "'");

And if you want to escape ', ", \, and nul (the byte null), you can use addslashes():

echo addslashes($value);