Migrating from PHP 5.5.x to PHP 5.6.x
PHP Manual

Deprecated features in PHP 5.6.x

Calls from incompatible context

Methods called from an incompatible context are now deprecated, and will generate E_DEPRECATED errors when invoked instead of E_STRICT. Support for these calls will be removed in a future version of PHP.

An example of such a call is: 

<?php
class A {
    function f() { echo get_class($this); }
}

class B {
    function f() { A::f(); }
}

(new B)->f();
?>

The above example will output:

Deprecated: Non-static method A::f() should not be called statically, assuming $this from incompatible context in - on line 7
B

$HTTP_RAW_POST_DATA and always_populate_raw_post_data

always_populate_raw_post_data will now generate an E_DEPRECATED error when $HTTP_RAW_POST_DATA is populated. New code should use php://input instead of $HTTP_RAW_POST_DATA, which will be removed in a future release. You can opt in for the new behaviour (in which $HTTP_RAW_POST_DATA is never defined hence no E_DEPRECATED error will be generated) by setting always_populate_raw_post_data to -1.

iconv and mbstring encoding settings

The iconv and mbstring configuration options related to encoding have been deprecated in favour of default_charset. The deprecated options are: