重新认识PHP中的set_time_limit

想必大家都已经知道set_time_limit函数的作用，即设置脚本最大执行时间。但是这个函数还是有一些细节有些人可能没太注意。

首先先看一下set_time_limit函数的解释。设置允许脚本运行的时间，单位为秒。如果超过了此设置，脚本返回一个致命的错误。默认值为30秒，或者是在php.ini的max_execution_time被定义的值，如果此值存在。当此函数被调用时，set_time_limit()会从零开始重新启动超时计数器。换句话说，如果超时默认是30秒，在脚本运行了了25秒时调用 set_time_limit(20)，那么，脚本在超时之前可运行总时间为45秒。

还有一个注意的地方是set_time_limit()函数和配置指令max_execution_time只影响脚本本身执行的时间。任何发生在诸如使用system()的系统调用，流操作，数据库操作等的脚本执行的最大时间不包括其中。同时sleep函数的时间也不包括在其中。

也就是说set_time_limit()函数只针对的是PHP代码本身的执行时间。

例子1：

set_time_limit(5);
sleep(10);
echo "test";

以上代码在10秒后正常的输出test,并且不会报错。
换成下面的代码试试

set_time_limit(5);
for(;;){
   //do something  
}

以上代码执行5秒后会报Fatal error: Maximum execution time of 5 seconds exceeded的错误。

例子2：

set_time_limit(5);
//假设curl需要10秒才能返回数据
$ch = curl_init(); 
curl_setopt($ch, CURLOPT_URL, "http://example.com"); 
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1); 
$output = curl_exec($ch);  
curl_close($ch);  
var_dump($output);

以上代码在10秒后正常的打印出curl请求返回的结果，并不会在5秒的时候报错。
同理，数据库的相关操作也是如此。

所以，看似一个简单的函数调用，有些细节上的东西还是很有必要了解的。不然坑的可就是自己了?。

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