当使用赋值运算符时不能回显字符串？

Question

当我用foreach运行变量数组时，我发现在使用加法时没有显示回波字符串，只使用operator.But回波赋值操作符，display.What是不能用加法运算符回显字符串的原因吗？

<?php
 $key = 3;
 echo $key+1;echo "<br>";      // 4
 echo "The answer is ".++$key; // The answer is 4
 echo "The answer is ".$key+1; // 1
 //last echo is why can't display string and not getting 4
?>

Answer 1

那是因为你在用字符串做数学运算。

"The answer is ".$key+1与

"The answer is 3" + 1 which equals 1;

您需要使用()来清除范围

$key = 3;
"The answer is ".($key+1) === "The answer is 4"

也是

echo "The answer is ".++$key; // The answer is 4
echo "The answer is ".($key+1); // This would be 5, beause you're incrementing $key by 1 beforehand