The new array syntax isn't a breaking change; however there was a lot of functionality deprecated in 5.3 that was completely removed in 5.4 which would definite

The new array syntax isn't a breaking change; however there was a lot of functionality deprecated in 5.3 that was completely removed in 5.4 which would definitely break some code. Unfortunately there were some bugs introduced in 5.4 that are preventing us from upgrading (our actual code is fine). Quite a shame as I'm quite looking forward to the new additions.