I am a sentence.

$sentence = explode(" ", $sentence);
/*
returns array(4) {
  [0]=>
  string(1) "I"
  [1]=>
  string(2) "am"
  [2]=>
  string(1) "a"
  [3]=>
  string(8) "sentence."
}
*/

$sentence = preg_split("/[.?!s]/", $sentence);
/*
returns array(5) {
  [0]=>
  string(1) "I"
  [1]=>
  string(2) "am"
  [2]=>
  string(1) "a"
  [3]=>
  string(8) "sentence"
  [4]=>
  string(0) ""
}
*/

$sentence = preg_split("/(?<=w)bs*/", 'I am a sentence.');

array(5) {
  [0]=>
  string(1) "I"
  [1]=>
  string(2) "am"
  [2]=>
  string(1) "a"
  [3]=>
  string(8) "sentence"
  [4]=>
  string(1) "."
}

Array
(
    [0] => Claire
    [1] => ’s
    [2] => favorite
    [3] => sonata
    [4] => for
    [5] => piano
    [6] => is
    [7] => Mozart
    [8] => ’s
    [9] => Sonata
    [10] => no
    [11] => . 15
    [12] => in
    [13] => C
    [14] => Major
    [15] => .
)

$parts = preg_split("/s+|b(?=[!?.])(?!.s+)/", $sentence);

Array
(
    [0] => Claire’s
    [1] => favorite
    [2] => sonata
    [3] => for
    [4] => piano
    [5] => is
    [6] => Mozart’s
    [7] => Sonata
    [8] => no.
    [9] => 15
    [10] => in
    [11] => C
    [12] => Major
    [13] => .
)

preg_split( '/[^a-zA-Z0-9]+/', 'I am a sentence' );

array(4) {
  [0]=>
  string(1) "I"
  [1]=>
  string(2) "am"
  [2]=>
  string(1) "a"
  [3]=>
  string(8) "sentence"
}

preg_split( '/b[^a-zA-Z0-9]+b/', 'I am a sentence.' );

array(4) {
  [0]=>
  string(1) "I"
  [1]=>
  string(2) "am"
  [2]=>
  string(1) "a"
  [3]=>
  string(8) "sentence."
}