PHP equality comparison between map fields always returns true

[PierrickVoulet]

This is a followup issue of #7650

Comparing map fields in PHP always returns true, regardless of the contents:

$mf1 = new MapField(GPBType::INT32, GPBType::INT32);
$mf2 = new MapField(GPBType::INT32, GPBType::INT32);
$mf2[0] = 1;

// This fails with PHP Extension (C implementation) but passes with PHP Module (pure-PHP implementation)
$this->assertFalse($mf1 == $mf2);

This is only the case when using the C implementation. This should be considered as a bug because the pure-PHP and C implementations do not behave in the same way.