Capecitabine Combined With Cisplatin in Treating Patients With Locally Advanced or Metastatic Solid Tumors

DISEASE CHARACTERISTICS: * Histologically confirmed metastatic or locally advanced inoperable carcinoma of the upper gastrointestinal tract, head and neck, lung, breast, or carcinoma of unknown primary * Previously treated and/or resected primary tumors allowed * Hormone receptor status: * Not specified PATIENT CHARACTERISTICS: Age: * 18 and over Sex: * Not specified Menopausal status: * Not specified Performance status: * ECOG 0-2 Life expectancy: * At least 12 weeks Hematopoietic: * Absolute neutrophil count at least 1,500/mm\^3 * Platelet count at least 100,000/mm\^3 Hepatic: * Bilirubin less than 2 mg/dL\* * AST less than 3 times upper limit of normal (ULN)\* * Alkaline phosphatase no greater than 3 times ULN\* NOTE: \* Unless related to tumor (e.g., cholangiocarcinoma or hepatic metastases) Renal: * Creatinine no greater than 1.5 mg/dL OR * Creatinine clearance greater than 60 mL/min * BUN no greater than 30 mg/dL\* NOTE: \* Unless related to tumor (e.g., cholangiocarcinoma or hepatic metastases) Other: * No other medical condition that could interfere with oral medication absorption * No prior or concurrent malignancy except surgically cured carcinoma of the cervix or basal cell or squamous cell carcinoma skin cancer * Not pregnant or nursing * Negative pregnancy test * Fertile patients must use effective contraception * HIV negative PRIOR CONCURRENT THERAPY: Biologic therapy: * Not specified Chemotherapy: * At least 6 months since prior fluo…