为什么("00e0&" =="00e1")评估为true? [英] Why does ("00e0" == "00e1") evaluate as true?

问题描述

在PHP中，为什么以下语句的前两个结果为true?

In PHP, why do the first two of the following statement evaluate true?

<?php
    if("00e0" == "00e1") {
        echo 'matches (a)';
    } else {
        echo 'failed (a)';
    }

    if("00e1" == "00e9") {
        echo 'matches (b)';
    } else {
        echo 'failed (b)';
    }

    if("00e2" == "00ea") {
        echo 'matches (c)';
    } else { 
        echo 'failed (c)';
    }
?>

如果运行，将返回以下内容:

If run this will return the following:

matches (a)
matches (b)
failed (c)

与其他"00e(0-9)"字符串相比，"00e0"，"00e1"，"00e2" .."00e9"之间的任何字符串都为真.

Any string between "00e0", "00e1", "00e2" .. "00e9" will give true if compared with another "00e(0-9)" string.

推荐答案

这是因为将有效的浮点值的字符串解释为这样.

It's because the strings that are valid floating point values are being interpreted as such.

例如，00e0等同于0 x 100，00e9等同于0 x 109，两者都为零，因此彼此相等.

For example, 00e0 is equivalent to 0 x 100 and 00e9 is equivalent to 0 x 109, both of which are zero, and hence equal to each other.

但是，由于00ea不是 个有效的浮点数，因此对它的处理方式有所不同.

However, since 00ea is not a valid floating point number, it is being treated differently.

您可以通过以下方式看到类似的效果:

You can see a similar effect with:

echo "01e2" - "01e1";

之所以输出90，是因为它与1 x 102 - 1 x 101或100 - 10相同.

which outputs 90 because it's the same as 1 x 102 - 1 x 101, or 100 - 10.

PHP doco (斜体字)支持:

如果将数字与字符串进行比较，或者比较涉及数字字符串，则将每个字符串转换为数字，然后以数字方式进行比较.

If you compare a number with a string or the comparison involves numerical strings, then each string is converted to a number and the comparison performed numerically.

该段落链接到另一页解释了转化背后的规则，如果发生的话:

That paragraph links to another page which explains the rules behind conversion, should it happen:

如果字符串不包含任何字符."，"e"或"E"，并且数值适合整数类型限制(由PHP_INT_MAX定义)，则字符串将被评估为整数.在所有其他情况下，它将被视为浮点数.

如果您要避免这种行为，则在第一个链接中有一条注释，指出您应改用===:

If you want to avoid this behaviour, there's a note in that first link which states you should use === instead:

当比较===或！==时，不会进行类型转换，因为这涉及比较类型和值.

The type conversion does not take place when the comparison is === or !== as this involves comparing the type as well as the value.

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