  A zero-dimensional (resp. symbolic) flow is a suspension flow over a
zero-dimensional system (resp. a subshift). We show that any topological flow
admits a principal extension by a zero-dimensional flow. Following [Bur19] we
deduce that any topological flow admits an extension by a symbolic flow if and
only if its time-$t$ map admits an extension by a subshift for any $t\neq 0$.
Moreover the existence of such an extension is preserved under orbit
equivalence for regular topological flows, but this property does not hold more
true for singular flows. Finally we investigate symbolic extensions for
singular suspension flows. In particular, the suspension flow over the full
shift on $\{0,1\}^{\mathbb Z}$ with a roof function $f$ vanishing at the zero
sequence $0^\infty$ admits a principal symbolic extension or not depending on
the smoothness of $f$ at $0^\infty$.
