php-little pitfalls of weak typing
Aug 08, 2016 am 09:31 AMAn issue discussed in the group today, record it.
The origin of the problem is
var_dump(md5('240610708') == md5('QNKCDZO')); //true
Two md5 strings, compared with the comparison operator (==), they are actually equal of.
Output the following two values:
md5('240610708') //0e462097431906509019562988736854
md5('QNKCDZO') //0e8304004519934940580242199033 91
When I saw the result, I felt that it was intval at first glance, so 0 == 0 returned true.
After being reminded later, I found out that this was not the case.
php comparison operator:
http://php.net/manual/zh/language.operators.comparison.php
Example:
如果比較一個(gè)數(shù)字和字符串或者比較涉及到數(shù)字內(nèi)容的字符串,則字符串會(huì)被轉(zhuǎn)換為數(shù)值并且比較按照數(shù)值來(lái)進(jìn)行。此規(guī)則也適用于 switch 語(yǔ)句。當(dāng)用 === 或 !== 進(jìn)行比較時(shí)則不進(jìn)行類(lèi)型轉(zhuǎn)換,因?yàn)榇藭r(shí)類(lèi)型和數(shù)值都要比對(duì)。
<?php var_dump(0 == "a"); // 0 == 0 -> true var_dump("1" == "01"); // 1 == 1 -> true var_dump("10" == "1e1"); // 10 == 10 -> true var_dump(100 == "1e2"); // 100 == 100 -> true switch ("a") { case 0: echo "0"; break; case "a": // never reached because "a" is already matched with 0 echo "a"; break; } ?>
In other words, this equality is not because the two strings are intval, and become 0=0
but is numericized 0e4xxx == 0e8xxx, which is scientific notation, 0*10 to the nth power, that is, 0.0000 and 0.00000000
, so there will be equal results.
The above has introduced the small pitfalls of php-weak typing, including aspects of it. I hope it will be helpful to friends who are interested in PHP tutorials.

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