Why is my php variable not outputting with javascript alert? [closed] - php

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I have a PHP variable which I am trying to pass into a javascript function. Using pure PHP I am to echo the desired string output, but when I shove it into a variable and output it in javascript it doesn't work.
It seems javascript doesn't seem to see anything at all.
Here's the code:
<?php $a = get_post_thumbnail_id( $post -> ID ); ?>
<?php $img = wp_get_attachment_image_src( $a ); ?>
<?php $b = $img[0]; ?>
<script type="text/javascript">
var myVar = <?php echo $b; ?>;
alert(myVar);
</script>
Whilst this is just a test piece of code I'm trying to make work, the results of which I am trying to make work with something like this:
<?php $a = get_post_thumbnail_id( $post -> ID ); ?>
<?php $img = wp_get_attachment_image_src( $a ); ?>
<?php $b = $img[0]; ?>
<script type="text/javascript">
$(".imgWindow").backstretch("<?php echo $b; ?>");
</script>
There's clearly some underlining principal of PHP and Javascript I must be missing.
Enlighten me please. Help appreciated.

Answer 1

<?php $a = get_post_thumbnail_id( $post -> ID ); ?>
<?php $img = wp_get_attachment_image_src( $a ); ?>
<?php $b = $img[0]; ?>
<script type="text/javascript">
var myVar = '<?php echo $b; ?>';
alert(myVar);
</script>
You have to wrap the value in quotes for non-integer values.
Alternatively, assign like this:
var myVar = '<?=$b;?>';

Answer 2

if it's a string "image name" use quotes
var myVar = '<?php echo $b; ?>';