NOTE: This is a condensed excerpt from my forthcoming (2014) commentary on Philippians.
While some have suggested that Paul writes to the Philippians from Caesarea or Ephesus, I am persuaded the evidence points to the traditional view of Rome.
First, our earliest textual evidence supports a Roman origin. The second-century Marcionite Prologue reads in part that “the apostle praises [the Philippians], writing from Rome in prison through Epaphroditus.” Other early Christian manuscripts include similar notes as well.
Second, the description of Paul’s circumstances in Philippians fits what we know of his house arrest in Rome. The period of at least two years allows ample time for the various travels mentioned or implied in the letter (Phil 2:19-30). Interacting with local Christians (Phil 1:12-18b), receiving visitors (Phil 2:25), and coordinating the travel plans of his ministry partners (Phil 2:19-30) are all consistent with the freedom Paul had while under house arrest.
Third, the various “Roman” references are best explained if Paul is in Rome. Based on the context, the reference to the praitōrion in Philippians 1:13 is best understood as the imperial guard (an elite group of soldiers assigned to protect the emperor) rather than a governor’s palace. While it is true that the imperial guard could be found in various locations throughout the empire, by far the largest concentration was in Rome. Paul closes the letter by singling out believers in “Caesar’s household” who send their greetings (4:22). Given Philippi’s pride in being a miniature Rome, these reference are most naturally understood if Paul writes from Rome.
Withouth question the central objection to Paul writing Philippians from Rome is the distance. Estimates range from 700 to 1,200 miles depending on the route taken. Travel times varied significantly depending whether one travelled by sea or land and the weather at a given time of the year. Further variables included the pace at which one walked or whether one was able to use a horse or other animals. All such factors make it difficult to categorically state how long a trip between Philippi and Rome might take. Perhaps the shortest route would have been to take the Via Egnatia (which passed through Philippi) west to Epidamnos/Dyrrhachium on the coast of Macedonia (about 350 miles) and make the 80 mile sea voyage across the Adriatic Sea to Brundisium. From there one could take the Via Appia some 350 miles to Rome. In the best conditions such a trip could be made by foot in about six weeks. In less favorable circumstances, it could take three months.
Of course, a number of possibilities could have significantly reduced the time period necessary for the progression of interaction between Paul and the Philippians. For example, word could have reached Philippi that Paul was being transferred to Rome before Paul left Caesarea or sometime during his difficult trip to Rome. At that point the Philippians could have dispatched Epaphroditus to meet Paul in Rome. Given that Epaphroditus carried a large sum of money to Paul it is extremely unlikely that he travelled alone. If Epaphroditus fell ill somewhere on the way to Rome, he could have sent someone from his travelling party back to Philippi to let them know while he and the rest of the team continued on to Rome. Based on that Epaphroditus could assume that his illness was known in Philippi and that his fellow believers would be worried about Epaphroditus’ well-being and his ability to serve Paul rather than being a burden.
Granted, such suggestions are necessarily speculative. But given the interconnectedness of the Pauline churches throughout the Mediterranean and the regular communication between these bodies, we must not underestimate how quickly news might travel.As such, the distance between Rome and Philippi does not exclude Rome as the place from which Paul writes Philippians.
 For the Latin text of the Marcionite Prologue, see Daniel J. Theron, Evidence of Tradition (Grand Rapids: Baker, 1980), 79-83. The translation here is my own.
 For example, at the end of Philippians in the fourth-century Codex Vaticanus, the scribe wrote “written from Rome.” Other manuscripts with similar notes include 075 (10th century), 1720 (10th century), 1739 (10th century), 424 (11th century), 1881 (14th century), and the Majority Text.
 See further Richard J. Cassidy, Paul in Chains: Roman Imprisonment and the Letters of St. Paul (New York: Crossroad Pub. Co., 2001), 124-35.
 For a helpful survey of the factors involved in travelling in the Roman Empire, see Lionel Casson, Travel in the Ancient World (Toronto: Hakkert, 1974), 149-96.
 For an excellent discussion see Michael B. Thompson, “The Holy Internet: Communication between Churches in the First Christan Generation,” in The Gospels for All Christians: Rethinking the Gospel Audiences (ed. Richard Bauckham; Grand Rapids: Eerdmans, 1998), 49-70.