What has changed in PHP 5.6.x
New features
Migrating from PHP 5.5.x to PHP 5.6.x
PHP Manual

## Backward Incompatible Changes

Although most existing PHP 5 code should work without changes, please take note of some backward incompatible changes:

### json_decode strictness

json_decode now rejects non-lowercase variants of the JSON literals true, false and null at all times, as per the JSON specification, and sets json_last_error accordingly. Previously, inputs to json_decode that consisted solely of one of these values in upper or mixed case were accepted.

This change will only affect cases where invalid JSON was being passed to json_decode: valid JSON input is unaffected and will continue to be parsed normally.

### GMP resources are now objects

GMP resources are now objects. The functional API implemented in the GMP extension has not changed, and code should run modified unless it checks explicitly for a resource using is_resource or similar.