Xdebug's, and your's, assumption that you should escape the \ already when doing property_get in step 3 is actually wrong:

<- property_get -i 18 -n $this->TestA\TestB\TestC\Aitems -d 0 -c 0 -p 0

It should be either:

<- property_get -i 18 -n "$this->TestA\TestB\TestC\Aitems" -d 0 -c 0 -p 0

or

<- property_get -i 18 -n $this->TestA\TestB\TestC\Aitems -d 0 -c 0 -p 0

Xdebug does currently not do this correctly either. The DBGp specs say (in https://xdebug.org/docs-dbgp.php#message-packets):

If a value for an option includes a space, the value needs to be encoded.
You can encode values by encasing them in double quotes:

property_get -i 5 -n "$x['a b']" -d 0 -c 0 -p 0

It is also possible to use escape characters in quoted strings:

property_get -i 6 -n "$x[\"a b\"]" -d 0 -c 0 -p 0

As Xdebug has returned, through XML, just:

fullname="$this->TestA\TestB\TestC\Aitems"

which, after XML decoding shows:

$this->TestA\TestB\TestC\Aitems

There is no space in this, so there is no NEED to enclose them in "'s, or does the spec say anything about escaping the \'s.

The spec hints that if you enclose them in "'s, you must also escape the \'s.

I propose we do the following:

• Xdebug stops stripping \'s when it parsing the property names, unless they are enclosed in "'s.
• PhpStorm does not escape the \'s, unless it has wrapped the value in double quotes. It must simply use the XML decoded value that XML returned (through "context_get" or "property_get -n $this") as argument to property_get unless there is a space or \0 character, in which case PhpStorm must enclose the value in "'s and escape ", \ and NULL.
• I update the DBGp spec to:
  • include the \0 character with the line about the space
  • make it clearer when you must escape
  • make it clearer when you can escape

This is likely going to break existing behaviour, so you might want to change behaviour depending on Xdebug version, send to PhpStorm in the init package with:

<engine version="2.6.0beta2-dev">

I've added a PR for DBGp, please review: https://github.com/derickr/dbgp/pull/17

Looks good for me. From IDE side the behaviour is the same as for Xdebug 2.5 and earlier.

That's not 100% true - please see this again:

Your assumption that you should escape the \ already when doing
property_get in step 3 is actually wrong:

<- property_get -i 18 -n $this->TestA\TestB\TestC\Aitems -d 0 -c 0 -p 0

It should be either:

<- property_get -i 18 -n "$this->TestA\TestB\TestC\Aitems" -d 0 -c 0 -p 0

or

<- property_get -i 18 -n $this->TestA\TestB\TestC\Aitems -d 0 -c 0 -p 0

Do you mean Xdebug 2.5 doesn't require escaping inside quotes?

For me, the command with escaped value works perfectly fine with Xdebug 2.5.5 + PHP 7.1.1 or PHP 5.6.30

<- property_get -i 23 -n "$this->PhpStormBug917319\ParentClass_attributes['Three\s Stuff']" -d 0 -c 0 -p 0
-> <response xmlns="urn:debugger_protocol_v1" xmlns:xdebug="http://xdebug.org/dbgp/xdebug" command="property_get" transaction_id="23"><property name="$this->PhpStormBug917319\ParentClass_attributes['Three\s Stuff']" fullname="$this->PhpStormBug917319\ParentClass_attributes['Three\s Stuff']" type="array" children="1" numchildren="1" page="0" pagesize="100"><property name="id" fullname="$this->PhpStormBug917319\ParentClass_attributes['Three\s Stuff']['id']" type="string" size="13" encoding="base64"><![CDATA[VGhyZWVccyBTdHVmZg==]]></property></property></response>

Inside double quotes, you MUST escape, like you do:

property_get -i 23 -n "$this->PhpStormBug917319\ParentClass_attributes['Three\s Stuff']"

And I showed with:

property_get -i 18 -n "$this->TestA\TestB\TestC\Aitems" -d 0 -c 0 -p 0

Not using double quotes, you MUST NOT escape, like I showed with:

<- property_get -i 18 -n $this->TestA\TestB\TestC\Aitems -d 0 -c 0 -p 0

In the original description of the bug, you wrote:

3) Call 'property_get' command for 'TestA\TestB\TestC\Aitems'.
<- property_get -i 18 -n $this->TestA\TestB\TestC\Aitems -d 0 -c 0 -p 0

But this uses escaped \ characters outside double quotes, which is incorrect.

Xdebug 2.5 would allow this, but should not have. Xdebug 2.6 will not allow this, after my merge of https://github.com/xdebug/xdebug/commit/307009a46ff63f15ea6e1e1e3c66637314089ab8

I understand that your suggestion in comments is different from the issue description.

I'm saying that Xdebug 2.5 does follow the same rules, e.g. The command with escaped blackslashes will fail in Xdebug 2.5 too:
<- property_get -i 22 -n $this->PhpStormBug917319\ParentClass_attributes -d 0 -c 0 -p 0
-> <response xmlns="urn:debugger_protocol_v1" xmlns:xdebug="http://xdebug.org/dbgp/xdebug" command="property_get" transaction_id="22" status="break" reason="ok"><error code="300"><message><![CDATA[can not get property]]></message></error></response>