Thanks for sharing your code.
I may be mistaken, but I believe that I can add a simplification to ituk’s post. If you just want the first item in the repeater, you can target it with the [0] array index without needing to reset the repeater field. So the following chunk of ituk’s code:
$repeater_field = $_POST['acf']['field_57bc841e7f973']; //repater parent key
reset($repeater_field); //reset repeater array to the rist row
$first_field = key($repeater_field); //get first key in the repeater array
$my_text_field = $_POST['acf']['field_57bc841e7f973'][$first_field]['field_57bc8a5447d06']; //specific field
can be simplified to this:
$my_text_field = $_POST['acf']['field_57bc841e7f973'][0]['field_57bc8a5447d06']; //specific field
NOTE: This only works for the 1st item! It seems that the first item uses the [0] array index but each repeater subfield that gets added gets a randomly generated array index. So instead of going to [1] for the second item it will be something like [58b641f67f3de], which is a randomly generated string that is probably generated from the server’s timestamp.
