Passing PHP variables to JavaScript

I was recently working on a project in CodeIgniter and I was setting numeric variables with constants for things like locations (1 = Maine, 2 = New Hampshire, etc.). This all worked fine but when it came to doing some JavaScript AJAX work I didn’t have access to these constants. One of the advantages to using these constants is that I did not want to have to change the value in more than one place if a value changed or I wanted to add a new location, for example. My way to solve this was to pass my PHP variables through to JavaScript using json_encode.

In my CodeIgniter controller, I set the variables I wanted to use in JavaScript:

$footerData['javascriptVariables'] = array(
  'sheetID' => $sheetID
);

Then, in the footer, I checked for the existence of $javascriptVariables and, if it is not empty, I echoed out the json_encoded value:

// Were there any variables set via PHP for use in the JavaScript?
if (!empty($javascriptVariables)) {
  echo '<script type="text/javascript">';
  echo "var php_variables = " . json_encode($javascriptVariables) . "\n";
  echo '</script>';

This results in something like:

<script type='text/javascript'>
    var php_variables = {"sheetID":"8"}
</script>

Then, in my JavaScript, I can easily get access to the sheetID set in PHP:

php_variables.sheetID

I suspect that I could also automate the process of making all my PHP constants available in JavaScript by using get_defined_constants.