Depends on what you want.
If you want to use the mouse:
player.radians = Math.atan2(_ymouse-player._y,_xmouse-player._x);
player._rotation = player.radians*180/Math.PI;
player._x += Math.cos(player.radians)*player.speed;
player._y += Math.sin(player.radians)*player.speed;
Otherwise, you can just use the conversion from degrees to radians from the rotation:
player.radians = player._rotation/180*Math.PI;
player._x += Math.cos(player.radians)*player.speed;
player._y += Math.sin(player.radians)*player.speed;
And a clarification on conversions:
Degrees to radians: Divide by 180 Multiply by PI
Radians to degrees: Multiply by 180 Divide by PI