You need to use stripslashes
to remove \\
and then decode
$data="[{\\"field_display_txt\\":\\"New custom field phone\\",\\"field_value\\":0},{\\"field_display_txt\\":\\"New custom field\\",\\"field_value\\":\\"test_zapier\\"},{\\"field_display_txt\\":\\"New custom field select\\",\\"field_value\\":\\"1\\"}]";
$json = json_decode(stripslashes($data), true);
print_r($json);
http://sandbox.onlinephpfunctions.com/code/a43301dddeb89aedca6a50eed703f935ac721496
solved getting Invalid json error when I am decoding below data using json_decode in PHP [closed]