<?php
$str = 'Fry me a Beaver. Fry me a Beaver! Fry me a Beaver? Fry me Beaver no. 4?! Fry me many Beavers... End';
var_export(
preg_split('~[.?!]+\K\s+(?=[A-Z])~', $str, 0, PREG_SPLIT_NO_EMPTY)
);
array (
0 => 'Fry me a Beaver.',
1 => 'Fry me a Beaver!',
2 => 'Fry me a Beaver?',
3 => 'Fry me Beaver no. 4?!',
4 => 'Fry me many Beavers...',
5 => 'End',
)