谁能告诉我这个问题怎么写

<?php 

echo $m1 = memory_get_usage(); 

echo "<br />";

echo $m2 = memory_get_usage()-$m1; 

echo "<br />";

echo $m3 = memory_get_usage()-$m1-$m2; 

echo "<br />";

echo $m4 = memory_get_usage()-$m1-$m2-$m3; 

echo "<br />";

echo $m5 = memory_get_usage()-$m1-$m2-$m3-$m4; 

?>

怎么写