Description:
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I don't know whether that's intentional behavior or not but the $this value can be overwritten with a variable variable (see test code below).

Test script:
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<?php

$var = 'this';

$$var = 10;

echo $this; // Echos 10

// Generates a fatal error
$this = 10;

Expected result:
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A PHP Fatal error message.

Actual result:
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$this gets silently overwritten.