Why does PHP consider 0 to be equal to a string?

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You are doing == which sorts out the types for you.

0 is an int, so in this case it is going to cast 'e' to an int. Which is not parsable as one and will become 0. A string '0e' would become 0 and would match!

Use ===

From PHP.net:

Comparisons between strings and numbers using == and other non-strict
comparison operators currently work by casting the string to a number,
and subsequently performing a comparison on integers or floats. This
results in many surprising comparison results, the most notable of
which is that 0 == “foobar” returns true.

However this behavior was changed in PHP 8.0:

When comparing to a numeric string, PHP 8 uses a number comparison.
Otherwise, it converts the number to a string and uses a string
comparison.

PHP 7

0 == 'foobar' // true
0 == '' // true
4 == '4e' // true (4e is cast as a number and becomes 4)

PHP 8 converts numbers to strings before making comparisons

0 == 'foobar' // false
0 == '' // false
4 == '4e' // false ('4e' is considered non-numeric therefore 4 is cast as a string and becomes '4')

This is a major change therefore it was implemented in a new major PHP version. This change breaks backward compatibility in scripts that depend on the old behavior.

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