If John Williams is an alien presence on traditional symphonic programs, his concertos and overtures are like the friendly UFOs visiting Earth in Close Encounters of the Third Kind: They may not entirely fit in, but that's what makes their presence interesting.

The composer has been the consistent musical voice of filmmaker Steven Spielberg for more than 40 years, an association that accounts for many of his 50 Oscar nominations and 22 Grammy Awards. For just as long, though, Williams has been writing classical concert works with an increasing assurance that's likely to be apparent in the Philadelphia Orchestra's John Williams mini-festival, which takes place over the next two weeks at the Kimmel Center.

"I wrote them for rejuvenation after the strictures of writing film music, which we all know about," Williams, 84, said before an editing session for his latest Spielberg film, The BFG (as in "big friendly giant"), scheduled for July 1 release. "I haven't expected a lot of performances from them. At least I didn't initially."

Then came French conductor Stéphane Denève, principal guest conductor of the Philadelphia Orchestra. His fascination with Williams' music began with the 1982 film E.T. Denève put excerpts from Close Encounters of the Third Kind on a symphonic subscription program last season that might ordinarily have been on a pops concert - and we have this larger tribute.

Concerts Thursday through Sunday will feature Yo-Yo Ma playing the Williams Cello Concerto, followed by concerts May 5-7 featuring the Violin Concerto with James Ehnes - both mixed in with Beethoven and Ravel. A one-night-only May 4 program of film music has Williams himself and Denève sharing the conducting platform.

Denève says that he has great personal interest in many kinds of what he calls "narrative music" - Williams', in particular - but that his concerts aren't necessarily an exercise in genre mixing. "When I do a program," he says, "I believe in each piece of music . . . that it's the highest quality and can stand by itself."

Greater appreciation for American film music in general seems to be welling up, often from Europe. Erich Wolfgang Korngold's Violin Concerto used to be dismissed as recycled movie music (which it is). Now, it is loved for that reason.

"There's still some condescension," says Williams. He cautions enthusiastic conductors against putting some of his more functional film music in a concert. Not all film composers know the difference. And even the greatest among them - such as Bernard Herrmann - didn't successfully transition into concert works. Denève describes Williams' output on that front as "first-class . . . with an incredible list of concertos." Ten, actually.

The Cello Concerto was written for Ma in 1994, and although the star cellist tends to premiere works and then leave subsequent dissemination to others, he's returning to this one. Also, Ma's schedule tends to be one-night galas - but not this week.

Violinist Ehnes, whose busy schedule doesn't easily accommodate major repertoire additions, learned Williams' 1974 Violin Concerto at Denève's instigation, and, in contrast to the primary-colors Star Wars music, this piece has "a lot of darkness. . . . It's not designed to be a pleasant violin concerto. It very pointedly has something it wants to say."

Ehnes first performed it in the Philadelphia Orchestra's 2015 season in Vail, Colo. In that program (and unlike the forthcoming Kimmel concerts), the concerto was surrounded by film music for a typical bring-the-kids summer audience. "I was a little bit worried," Ehnes said. "But within a few minutes, they realized, 'Oh, same voice speaking a different language.' "

Clearly, Williams intimately knew the great 20th-century composers when he wrote his Violin Concerto, paying homage to Prokofiev and tipping a hat to Korngold. But the final movement goes to a more personal, mournful place. Then you discover the concerto is dedicated to his first wife, actress Barbara Ruick, who died from a cerebral hemorrhage in March 1974.

The impact of that event extended well beyond the concerto. Williams' emergence from a supercompetent worker bee in the Hollywood music mills into the John Williams we now know dates from that year - and the score for Jaws, whose distilled two-note motif became an iconic sound of implacable menace.

Creative breakthroughs in the face of personal trauma aren't unusual in the lives of artists. "It makes or breaks people," Williams said. "I was young and my wife was very young. Had I been 20 years older, I would've been less able to restart my life. Immediately thereafter, I did feel differently about working, especially with music. I can't quantify or describe what it did. But we do change. All life experience does that. We have accidents. We have happy things. Having three teenage children at the time required a lot of focus and strength, being a single parent. . . . We could talk for a week about this."

Williams came from a family with hardworking New England roots, but he grew up in Los Angeles, with training that included the great classical pianist Rosina Lhévinne at the Juilliard School and studio work with Henry Mancini. He wrote music for the TV show Lost in Space and adapted the music of others for the film version of Fiddler on the Roof.

Williams credits a lot of his success with the opportunities that came with Spielberg. Documentary films show the director working extremely collaboratively with Williams, and even recutting a scene to fit the score. During Williams' 1980-93 tenure as music director of the Boston Pops Orchestra, audiences often were the first to hear forthcoming scores to high-profile films.

But while everyone remarks at Williams' modesty and his superb, courtly manners, he did walk out on a Boston rehearsal when musicians hissed one of his pieces. So he stands up for his music, though his self-critical nature drives him to revise concertos many times.

Now he's one of the few 84-year-olds working in the thick of the film industry. At one point in the last decade, there was a three-year hiatus. Williams says he can't remember why. "Music," he says, "is something you don't leave."