Notice: Array to string conversion

Uh, this is weird. Looking at the source I cannot really tell what happens. Could you give some more information on the server software you are using, as well as on the PHP version?

The relevant part from the authcache.php frontcontroller script:

  20 // Relocate contents of $_SERVER variable to DRUPAL_ROOT by removing path
  21 // components pointing to the authcache front controller.
  22 $preg = '|' . preg_quote(substr($__file__, strlen($drupalroot) + 1), '|') . '|';
  23 $_SERVER = preg_replace($preg, 'authcache.php', $_SERVER);

The array-to-string warning origins at the line 23, however the only array parameter there is $_SERVER. Also preg_replace should operate on arrays even when $pattern and $replacement parameters are strings.

If you find the root cause which results in this warning, I'd be very interested in knowing about.

If you run into this on a production system, I recommend to use the safe_frontcontroller/authcache.php script instead. Copy it to the drupal root directory and follow the instructions given in the docblock. This will eliminate the need to dynamically discover the root directory and tamper with $_SERVER global completely.

I am getting the same error. I tried implementing:

If you run into this on a production system, I recommend to use the safe_frontcontroller/authcache.php script instead. Copy it to the drupal root directory and follow the instructions given in the docblock. This will eliminate the need to dynamically discover the root directory and tamper with $_SERVER global completely.

but, I had no luck and Notice is still showing.
I am using MAMP localhost
Server info:
Apache
PHP 5.3.27

I don't really know why the error is happening, but to make it go away, I looped through the $_SERVER variable and did the replace on each item. I attached a patch if interested.

@ryan88: Very interesting. This indicates that under some circumstances there can be an entry on the $_SERVER variable which is not a string but an array. Can you identify the entry which causes this notice?

According to the PHP documentation, the argv key contains an array, but it should only be present when running from the command line (see this SO answer).

Do you have the vars-entry on your $_SERVER or is it something different?

I have the same problem when running on local MAMP stack, I tried on 5.3.20 & 5.4.10.

Here is the dump you've asked, the array is indeed in 'argv':

Array
(
    [protossl] => 
    [HTTP_AUTHORIZATION] => 
    [HTTP_HOST] => v***p.be.dev
    [HTTP_CONNECTION] => keep-alive
    [HTTP_ACCEPT] => */*
    [HTTP_X_REQUESTED_WITH] => XMLHttpRequest
    [HTTP_USER_AGENT] => Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_3) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/35.0.1916.114 Safari/537.36
    [HTTP_X_AUTHCACHE] => 1
    [HTTP_REFERER] => http://v***p.be.dev/
    [HTTP_ACCEPT_ENCODING] => gzip,deflate,sdch
    [HTTP_ACCEPT_LANGUAGE] => nl-NL,nl;q=0.8,en-US;q=0.6,en;q=0.4,af;q=0.2,ca;q=0.2,de;q=0.2,es;q=0.2,fr;q=0.2,it;q=0.2,pl;q=0.2
    [HTTP_COOKIE] => cookie-agreed-nl=2; has_js=1; __utma=97863512.978894125.1401264938.1401264938.1401264938.1; __utmb=97863512.17.10.1401264938; __utmc=97863512; __utmz=97863512.1401264938.1.1.utmcsr=(direct)|utmccn=(direct)|utmcmd=(none)
    [HTTP_IF_NONE_MATCH] => "1401266099"
    [HTTP_IF_MODIFIED_SINCE] => Wed, 28 May 2014 08:34:59 +0000
    [PATH] => /usr/bin:/bin:/usr/sbin:/sbin
    [SERVER_SIGNATURE] => 
    [SERVER_SOFTWARE] => Apache
    [SERVER_NAME] => v***p.be.dev
    [SERVER_ADDR] => 127.0.0.1
    [SERVER_PORT] => 80
    [REMOTE_ADDR] => 127.0.0.1
    [DOCUMENT_ROOT] => /Applications/MAMP/htdocs/v***p
    [SERVER_ADMIN] => you@example.com
    [SCRIPT_FILENAME] => /Applications/MAMP/htdocs/v***p/sites/all/modules/authcache/modules/authcache_p13n/frontcontroller/authcache.php
    [REMOTE_PORT] => 51078
    [GATEWAY_INTERFACE] => CGI/1.1
    [SERVER_PROTOCOL] => HTTP/1.1
    [REQUEST_METHOD] => GET
    [QUERY_STRING] => a=&r=frag/block/views-shopping_cart-block&o[q]=&v=null
    [REQUEST_URI] => /sites/all/modules/authcache/modules/authcache_p13n/frontcontroller/authcache.php?a=&r=frag/block/views-shopping_cart-block&o[q]=&v=null
    [SCRIPT_NAME] => /sites/all/modules/authcache/modules/authcache_p13n/frontcontroller/authcache.php
    [PHP_SELF] => /sites/all/modules/authcache/modules/authcache_p13n/frontcontroller/authcache.php
    [REQUEST_TIME_FLOAT] => 1401266775.77
    [REQUEST_TIME] => 1401266775
    [argv] => Array
        (
            [0] => a=&r=frag/block/views-shopping_cart-block&o[q]=&v=null
        )

    [argc] => 1
)

Fixed, thank you.