Charlie Casey, or the youtuber charlieskies, became friends with Phil some time in mid to late 2008. The oldest currently surviving tweet from Phil to Charlie is dated 4 Nov 2008, in which he references talking to Charlie.
They communicated frequently per Twitter, Dailybooth, Skype and phone and they had a collab channel together called pabloislove. They met in March of 2009 and filmed a video that Phil at some point removed from his channel prior to 2012. No one appears to have saved the video before deletion. The Zoo Day video thumbnail can be seen on the wayback machine [link] and his tweet promoting the video remains online.
Charlie and Phil's Apple Store photos from the zoo day:
There is nothing definitive stated by either of them at the time to indicate a relationship, and they seemed to fall out of constant contact within a matter of months after meeting in real life. In June of 2009, around the same time Dan and Phil first began talking, Charlie began dating a mutual friend Stephen (3sixty5days). The four of them were friends up until Charlie and Stephen's messy breakup in 2010 that resulted in Charlie breaking Stephen's nose.
In the 2011-2012 period, Charlie made a few comments about his relationship with Phil, though ultimately none were entirely clear and Phil has certainly never offered up any perspective from his end.
In 2012 and 2013 Charlie also became increasingly hostile towards Dan on social media and tweeted a number of inflammatory things - joking about phan, Dan's sexuality, calling Dan an asshole, and similar things. After a series of especially confrontative tweets in 2013 Dan removed Charlie as a friend on all his social media accounts.
Charlie continued to discuss Dan and Phil on twitter and in liveshows, alternating defensive/appreciative tones with Phil and derogatory tones when it came to Dan or Dan and Phil together. Phil unfollowed him on social media in early 2014.
Charlie has since kept quiet about D&P and withdrew completely from social media in 2015.
Emma Blackery briefly mentions Charlie and his relationship with Dan and Phil twice in 2016:
Her support of the charlieskies twitter bullying on her main channel (11:49):
And in July 2016 (the 26th) she was invited to speak in Shane Dawson's podcast and said Dan and Phil "hate" Charlie now:"I used to laugh along with a friend who was really nasty to Dan and Phil. I used to think it was fun to sort of show tweets of support to that and, and feed off that drama, that's what I used to do, I used to be like that. And looking at it now I'm not even that person now, it's weird that that's me I don't, I don't think in those negative ways anymore."
transcribing creds to fancybum!Shane: So who have you had beef with?
Emma: This is the thing, I don't like saying because every time I even say their names, their massive group of fans-- massive group of fans-- of these two Youtubers--
Shane: Ohhh-- OHHH!
Emma: They hate me, they don't like me, their fans. At all.
Shane: Ok, I won't say their names.
Emma: Some of them do. It's a vocal minority that really don't like me. I was a massive fangirl of these two Youtubers who were very big in the UK, a massive fangirl back in 2012-13. They're one of the reasons that I started, I think they're great people. Um. But I was very over-the-top with my fangirliness; I had the shirts, I'd make all the references, I was basically cringe-worthy. It was cringetastic. And I really wanted to be friends with them, and I was trying to talk to them, I'd email them to collab, I had like 10K subscribers, right. I was very naive and stuff, and like obviously they would get so many emails, they didn't reply or anything. But I took it personally, and this was years ago. I took it personally, like "Wow, they're shading me. They don't like me." And I was friends with someone who-- uhh... was... friends with them a long time ago who they now hate so then lots of drama started with that. And ever since--