Ecco un esempio:
<?php
$mystring = 'La {mia casa} è {molto} grande';
preg_match_all('/(?<={)(.*?)(?=})/', $mystring, $matches);
var_dump($matches);
/* Output:
array(2) {
  [0]=>
  array(2) {
    [0]=>
    string(8) "mia casa"
    [1]=>
    string(5) "molto"
  }
  [1]=>
  array(2) {
    [0]=>
    string(8) "mia casa"
    [1]=>
    string(5) "molto"
  }
}
*/
source: 
https://stackoverflow.com/a/1454936/874578