Why does PHP convert a string with the letter E into a number?

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"608E-4234" is the float number format, so they will cast into number when they compares.

608E-4234 and 272E-3063 will both be float(0) because they are too small.

For == in php,

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.

http://php.net/manual/en/language.operators.comparison.php

Attention:

What about the behavior in javascript which also has both == and ===?

The answer is the behavior is different from PHP. In javascript, if you compare two value with same type, == is just same as ===, so type cast won’t happen for compare with two same type values.

In javascript:

608E-4234 == 272E-3063 // true
608E-4234 == "272E-3063" // true
"608E-4234" == 272E-3063 // true
"608E-4234" == "272E-3063" // false (Note: this is different form PHP)

So in javascript, when you know the type of the result, you could use == instead of === to save one character.

For example, typeof operator always returns a string, so you could just use

typeof foo == 'string' instead of typeof foo === 'string' with no harm.

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