PHP7中如何使用太空船操作符

$c= $a <=> $b;

<?php
   //整型比较
   print( 1 <=> 1);print("<br/>");                    //0
   print( 1 <=> 2);print("<br/>");                    //-1
   print( 2 <=> 1);print("<br/>");                    //1
   print("<br/>");
   //字符型比较
   print( 1.5 <=> 1.5);print("<br/>");//0
   print( 1.5 <=> 2.5);print("<br/>");//-1
   print( 2.5 <=> 1.5);print("<br/>");//1
   print("<br/>");
   //字符型
   print( "a" <=> "a");print("<br/>");//0
   print( "a" <=> "b");print("<br/>");//-1
   print( "b" <=> "a");print("<br/>");//1
?>