Description:
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In any context impossible to override $this, so this code is incorrect:

$this = 123;

But possible to rewrite $this with this example:

$t = 'this';
$$t = 123;
var_dump($this); // int(123)

Test script:
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$t = 'this';
$$t = 123;
var_dump($this); // int(123)

Expected result:
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Runtime error

Actual result:
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No runtime error