$str = "Hello world. It's a beautiful day.";
$kelimeler = explode(" ", $str);

echo $kelimeler[0]."<br>";                 
echo $kelimeler[1]."<br>";                 
echo $kelimeler[2]."<br>";                
echo $kelimeler[3]."<br>"; 
echo $kelimeler[4]."<br>";                
echo $kelimeler[5]."<br>";