distinguish between null variable and unset variable



How can I tell the difference between a variable whose value is null and
a variable which is not set?

// cannot use === null:

ket% php -r '$null = null; var_dump(null === $null);'
bool(true)
ket% php -r 'var_dump(null === $unset);'
bool(true)
ket%

// - cannot use isset() either:

ket% php -r '$null = null; var_dump(isset($null));'
bool(false)
ket% php -r 'var_dump(isset($unset));'
bool(false)
ket%

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