[PHP] substr占用内存谨慎使用

            $header_size = curl_getinfo($curl_handle, curlinfo_header_size);
            $this->response_headers = substr($this->response, 0, $header_size);
            $this->response_body = substr($this->response, $header_size);
            $this->response_code = curl_getinfo($curl_handle, curlinfo_http_code);
            $this->response_info = curl_getinfo($curl_handle);

<?php
$start=memory_get_usage();
$str="123";
var_dump(memory_get_usage()-$start);
//$a=substr($str,0,1);
$a=$str;
var_dump(memory_get_usage()-$start-24);
$b=$str;
//$b=mb_substr($str,1);
var_dump(memory_get_usage()-$start-24);
$c=$str;
//$b=mb_substr($str,1);
var_dump(memory_get_usage()-$start-24);

<?php
$start=memory_get_usage();
$str="123";
var_dump(memory_get_usage()-$start);
//$a=substr($str,0,1);
$a=$str;
var_dump(memory_get_usage()-$start-24);
//$b=$str;
$b=substr($str,0,1);
var_dump(memory_get_usage()-$start-24);
//$c=$str;
$c=substr($str,1);
var_dump(memory_get_usage()-$start-24);