Why does ("00e0" == "00e1") evaluate as true?

Question

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)';
    }
?>

If run this will return the following:

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

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

Answer 1

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

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.

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

You can see a similar effect with:

echo "01e2" - "01e1";

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

This is supported by the PHP doco (my italics):

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.

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

If the string does not contain any of the characters '.', 'e', or 'E' and the numeric value fits into integer type limits (as defined by PHP_INT_MAX), the string will be evaluated as an integer. In all other cases it will be evaluated as a float.

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

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