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You can use solvePnP() to calculate euler angles , but the out put of solvePnP() is not directly roll, yaw & pitch, but need to calculate using rotation matrix.

I also came through similar problem, and found the function decomposeProjectionMatrix which gives euler angles as it's output.

In C++ the implementation should be something like below, assumes you already get the rotation Mat from solvePnP() like,

solvePnP(..,...,rotCamerMatrix,transMatrix,false);
Rodrigues(rotCamerMatrix,rotCamerMatrix1);

Now using the function,

void getEulerAngles(Mat &rotCamerMatrix,Vec3d &eulerAngles){
 Mat cameraMatrix,rotMatrix,transVect,rotMatrixX,rotMatrixY,rotMatrixZ;
 double* _r = rotCamerMatrix.ptr<double>();
 double projMatrix[12] = {_r[0],_r[1],_r[2],0,
                      _r[3],_r[4],_r[5],0,
                      _r[6],_r[7],_r[8],0};

 decomposeProjectionMatrix( Mat(3,4,CV_64FC1,projMatrix),
                           cameraMatrix,
                           rotMatrix,
                           transVect,
                           rotMatrixX,
                           rotMatrixY,
                           rotMatrixZ,
                           eulerAngles);
 }

Call it like,

 Vec3d eulerAngles;
 getEulerAngles(rotCamerMatrix1,eulerAngles);

where,

yaw   = eulerAngles[1]; 
pitch = eulerAngles[0];
roll  = eulerAngles[2];