Singer-actress is reportedly already planning the wedding

"American Idol" runner-up Katharine McPhee is definitely engaged ... or at least that's what one source insists.

Speculation about the betrothal has been swirling around the singer for months, and another unnamed source has come forward to tell People magazine that McPhee and her boyfriend, actor Nick Cokas, will tie the knot soon.

"Katharine is bubbling over with excitement for her upcoming wedding," the source says. "She is all smiles when she talks about the planning. It sounds like everything is going very smoothly."

McPhee has also been spotted showing off her ring from Cokas.

The couple met in 2005 when they were both performing in a Los Angeles production of "The Ghost and Mrs. Muir." Their friendship continued through her "Idol" experience, when he drove her to her San Francisco audition. After that, romance began to bloom between the 23-year-old from Sherman Oaks, Calif., and the 42-year-old.

So far, however, there has not been any official statment from McPhee's rep.

McPhee is delving into acting on the big screen, recently wrapping the comedy "I Know What Boys Like," playing a pregnant hippie. The film also stars Anna Faris, Rumer Willis, Emma Stone and Colin Hanks. She will also join Cokas, who will produce, for the upcoming indie film "The Last Caller," in which she plays a narcissistic woman searching for love.

McPhee's self-titled debut album dropped in January, debuting at No. 2 on the Billboard 200 chart.