Hola @juan
<?php
$text="hola @jose como esta @manuel?";
preg_match_all('/(@[\w]*)/', $text, $matches);
var_dump($matches);
?>
Array
(
[0] => Array
(
[0] => @jose
[1] => @manuel
)
[1] => Array
(
[0] => @jose
[1] => @manuel
)
)