php long numbers in string comparison

Question

var_dump("555555555555555555555" == "555555555555555555553"); //bool(true)
var_dump("aaaaaaaaaaaaaaaaaaaaa" == "aaaaaaaaaaaaaaaaaaaab"); //bool(false)

Why does this happen?

I know I can use

var_dump(strcmp("555555555555555555555", "555555555555555555553") == 0); //bool(false)

But why the first row returns true?

possible duplicate of What is the difference between == and === in php — mario
– mario, Commented Oct 3, 2013 at 14:24

Denis · Accepted Answer · 2013-10-03 14:21:41Z

4

It's a side effect of type-coercing. There's an article on phpsadness about it. Basically, the strings in the comparison are converted to numeric types, and due to precision loss, appear to be equal.

answered Oct 3, 2013 at 14:21

Denis

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2 Comments

Denis Over a year ago

@MarcelKorpel, absolutely. I have a strict rule to never use == at all.

Marcel Korpel Over a year ago

I'm just stuck by a higher PHP version. Always use === to compare those strings.

Naveen Kumar Alone · Accepted Answer · 2013-10-03 14:22:30Z

1

In your first row

var_dump("555555555555555555555" == "555555555555555555553");

it is true

Why because, the type-coercing comparison operators will coerce both operands to floats if they both look like numbers, even if they are both already strings

This bug is discussed here

answered Oct 3, 2013 at 14:22

Naveen Kumar Alone

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