  In the hot and dense hadronic sources formed in high-energy heavy-ion
collisions, the particle interactions in medium might lead to a squeezed
back-to-back correlation (BBC) of boson-antiboson pairs. We calculate the BBC
functions of $\phi\phi$ for sources evolving hydrodynamically in ($2+1$)
dimensions and with longitudinal boost invariance. The BBC functions for
hydrodynamic sources exhibit oscillations as a function of the particle
momentum because the temporal distributions of hydrodynamic sources have sharp
falls to 0 at large evolving times. The dependences of the BBC functions on the
directions of the particle momentum are investigated. For transverse
anisotropic sources, the BBC functions are minimum when the azimuthal angles of
the particles reach 0. The BBC functions increase with decreasing absolute
value of the particle pseudorapidity. The oscillations and the dependences on
the particle azimuthal angle and pseudorapidity are the significant signatures
for detecting the BBC in high-energy heavy-ion collisions.
