计算php函数的执行时间

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<?php
$long_str = "this is a test to see how much time md5 function takes to execute over this string";

// start timing from here
$start = microtime(true);

// function to test
$md5 = md5($long_str);

$elapsed = microtime(true) - $start;

echo "That took $elapsed seconds.\n";
?>