The French-born Mbolhi impressed during the 2014 World Cup in Brazil where Algeria reached the knock out stages.
The African side were beaten 2-1 by Germany after extra time but Mbolhi was named man of the match following a series of fine saves.
Mbolhi, the most capped foreign keeper in the Bulgarian league, joined CSKA from Russia's Krilya Sovetov in 2013, having already been loaned twice to the Sofia-based club.
The 28-year-old, who also played for CSKA's city rivals Slavia as well as clubs in Scotland, Greece, Japan and France, had a trial with the Premier League club Manchester United in 2010.
Philadelphia are seventh in the MLS's Eastern Conference with 23 points from 21 games.
(Reporting by Angel Krasimirov; Editing by Greg Stutchbury)
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