  Let $p>3$ be a prime, and let $m$ be an integer with $p\nmid m$. In the paper
we solve some conjectures of Z.W. Sun concerning
$\sum_{k=0}^{p-1}\binom{2k}k^3/m^k\pmod{p^2}$,
$\sum_{k=0}^{p-1}\binom{2k}k\b{4k}{2k}/m^k\pmod p$ and
$\sum_{k=0}^{p-1}\binom{2k}k^2\b{4k}{2k}/m^k\pmod {p^2}.$ In particular, we
show that $\sum_{k=0}^{\frac{p-1}{2}}\binom{2k}k^3\equiv 0\pmod {p^2}$ for
$p\equiv 3,5,6\pmod 7$. Let $P_n(x)$ be the Legendre polynomials. In the paper
we also show that $ P_{[\frac {p}{4}]}(t)\equiv
-\big(\frac{-6}{p}\big)\sum_{x=0}^{p-1}
\big(\frac{x^3-3/2(3t+5)x-9t-7}{p}\big)\pmod p$ and determine
$P_{\frac{p-1}{2}}(\sqrt 2), P_{\frac{p-1}{2}}(\frac{3\sqrt 2}{4}),
P_{\frac{p-1}{2}}(\sqrt {-3}),P_{\frac{p-1}{2}}(\frac{\sqrt 3}{2}),
P_{\frac{p-1}{2}}(\sqrt {-63}), P_{\frac{p-1}{2}}(\frac {3\sqrt 7}{8}) \pmod
p$, where $t$ is a rational $p-$integer, $[x]$ is the greatest integer not
exceeding $x$ and $(\frac {a}{p})$ is the Legendre symbol. As consequences we
determine $P_{[\frac {p}{4}]}(t)\pmod p$ in the cases $t=-5/3,-7/9,-65/63$ and
confirm many conjectures of Z.W. Sun.
