问题
I was just debugging a script and found that an if-statement wasn't working the way I expected it to.
var_dump("6064365413078728979" == "6064365413078728452");
die();
The code above will result in the following:
bool(true)
With the === operator it works as expected. Anyone got any ideas why?
I'm using PHP Version 5.3.13 with a wamp installation on a x64 windows machine.
回答1:
<?php
$a=6064365413078728979;
$b=6064365413078728452;
echo $a."<br>".$b;
//var_dump( $a==$b );
die();
?>
When you run that, then on your machine that might be exceeding limit for a number and that is a numeric comparison taking place. Try the script above and see value for $a will probably be different than the value you gave.
That is why when both are compared numerically they are equal. Hence use === as suggested by others
Edit: Explanation based upon @Axel's Advice.
PHP Manual explains
The size of a float is platform-dependent, although a maximum of ~1.8e308 with a precision of roughly 14 decimal digits is a common value (the 64 bit IEEE format).
And this website is offering and explanation on the Overflow phenomenon and a small php code to test your system's integer and float range. Getting to know the limit on your servers will most probably explain it best why the offerflow occured
回答2:
PHP has loose type comparison behavior, so your numerical strings are getting converted to integer types before == non strict comparison, and the conversion result is overflowing.
That is the principal reason to use === when it's possible.
Take a look at this page for further details on type juggling.
来源：https://stackoverflow.com/questions/14259162/comparing-different-strings-in-php-with-returns-true