Let’s look at important things here –
Mercury - Mercury represents communication skills, marketing, calculative ability, younger siblings, hobbies, skills with hands, intelligence, quick decision making, logical thinking, youthfulness etc.
Purva Phalguni - Purva Phalguni is nakshatra of Luxury, Wealth and Lavish Life. It is also nakshatra of creative pursuits and love/romance. For detailed information, please check this - https://www.astrosaxena.com/ppnaksh.
Leo - As Purva Phalguni is a part of Leo, Leo sign and its representations become important. Leo is 5th sign of zodiac belt, hence it represents the things associated with 5th house of horoscope like Love, Creativity, Arts, Being on Stage etc. Besides that, Leo is basically the Lion. This is the sign of feeling like King or Queen. Also, it is a sign of independence, Confidence, Ego, Self Esteem etc. Leo is consisted of 2 and half Nakshatra namely Magha, Purva-Phalguni and Uttara-Phalguni. Leo's lord is Sun..
Sun - As Sun rules Leo, Sun's position and dignity is important to know about the overall functioning of any planet in Purva Phalguni/Leo.
Venus - As Purva Phalguni is ruled by Venus, Venus's position and dignity will also have an important role in functioning and results of any planet in Purva Phalguni.
Houses Involved – Also, we can’t afford to forget the houses involved as a planet can be ruling different houses and sitting in different house for every person as per ascendant and planet’s position. To know overall affect, we have to consider the houses involved as those areas of life are being impacted.
Interpretation of Mercury in Purva Phalguni Nakshatra – Mercury is in friendly sign of Sun and friendly nakshatra of Venus here. So, dignity-wise Mercury is strong here. As Purva Phalguni is nakshatra related with creativity and manifests the creativity aspect of Leo and Mercury itself represents skills, interests and hobbies, this planetary position can create someone very creative and skillful. They can be good at writing, acting, music or any other type of creative interests. Not to forget that Leo itself is sign of creativity which makes this creativity aspect of life as most potential result with this position. It is very easy for them to gain out of their creative business and communication. They can also be very romantic. They can be more playful rather than "loyal to their partner" kind-of people as Mercury is flirtatious and is a frequent traveler. They can be very smart with money matters too. We can also see them working in financial fields. Otherwise also, they can be really smart with their financial dealings in personal life. Overall, a great position for creativity and finance related career but in matters of relation, they may not be trusted heavily.
Role of Sun – Sun placement is important to guide Mercury in right direction, being Leo's lord..
Role of Venus - Likewise, Venus placement is important to guide Mercury in right direction, being Purva Phalguni's lord.
Other Factors – And of course, we need to consider conjunction, aspects, house lordship, house position and dashas etc to know the exact results of Mercury in Purva Phalguni. So, we always need to take a holistic view of chart rather than just 1 planetary position.
Conclusion – So, this is how I see Mercury in Purva Phalguni can work in a chart.