HTML Header Content Security Policy not reporting to file - .htaccess

Question

After reading an introduction to CSP and the CSP website I wanted to get a feedback log of CSP breaches on my website.
So, I set up a header in my .htaccess as defined in the links above:
Header set Content-Security-Policy "default-src 'self' https://www.google-analytics.com https://www.jscache.com;"
This works, but for various reasons there are other entities that are still being blocked, so I want to setup a CSP report. Again, something that is clearly explained in the documents linked above.
And back to my .htaccess I replace the line with:
Header set Content-Security-Policy-Report-Only "default-src 'self'; report-uri /csp_report_parser.txt;"
Which I want to report all the non-self requests into a file, /csp_report_parser.txt . This file I have created on the same place as the .htaccess but the file fails to be populated, despite feedback from firebug stating:
Content Security Policy: The page's settings observed the loading of a resource at https://www.google-analytics.com/analytics.js ("default-src http://mywebsite"). A CSP report is being sent.
and many of these each time stating "A CSP report is being sent". The file this report should be sent to exists and I have given permissions 777 yet the file remains blank.
What have I missed to make this reporting feedback into the file stated?
I have also tried specifying an absolute URL to report to such as :
Header set Content-Security-Policy-Report-Only "default-src 'self'; report-uri http://mywebsite/csp_report_parser.txt;"
But again, this file on my domain remains blank despite browser firebug statements as above.

Answer 1

While I was expecting a data dump in a JSON format what I actually neeeded was to process the passed data to be dumped first, using a PHP json_decode function before saving the data to the file.
$data = json_decode(file_get_contents('php://input'), true);
file_put_contents('csp_report_parser.txt',print_r($data,true),FILE_APPEND);