Suppose a classical field configuration of a real scalar field $\phi(x,t)$, in $1+1$ dimensions, has the energy $$E[\phi]=\int\limits_{-\infty}^{+\infty} dx\, \left[\frac{1}{2}\left(\frac{\partial\phi}{\partial t}\right)^2+\frac{1}{2}\left(\frac{\partial \phi}{\partial x} \right)^2+a\phi^2+b\phi^4 \right]$$ where the potential is given by $$V(\phi)=a\phi^2+b\phi^4.$$
For the energy $E[\phi]$ to be finite, one necessarily requires $\phi\rightarrow 0$, as $x\rightarrow\pm\infty$. But in addition, shouldn't we also necessarily require that both $\frac{\partial\phi}{\partial t}$ and $\frac{\partial \phi}{\partial x}$ must vanish as $x\rightarrow \pm\infty$?
However, if I'm not mistaken, to find soliton solutions (which have finite energy), one imposes only the boundary condition $\phi\rightarrow 0$, as $x\rightarrow\pm\infty$. Does it mean that if this is satisfied, the vanishing boundary condition on the derivatives of the field are also automatically satisfied?
EDIT: I know that for arbitrary functions $f(x)$ this is not true i.e., if $f(x)\rightarrow 0$ as $x\rightarrow \pm \infty$, $f^\prime(x)$ need not vanish as $x\rightarrow \pm \infty$. But $\phi(x,t)$ are not arbitrary functions in the sense that they are solutions of Euler-Lagrange equations. Therefore, it may be possible that the condition $\phi\rightarrow 0$, as $x\rightarrow\pm\infty$ is sufficient.