We study the existence of solution to the system of differential equations $$(\phi (u'))'=f(t,u,u')$$
(
ϕ
(
u
′
)
)
′
=
f
(
t
,
u
,
u
′
)
with nonlinear boundary conditions $$\begin{aligned} g(u(0),u,u')=0, \quad h(u'(1),u,u')=0, \end{aligned}$$
g
(
u
(
0
)
,
u
,
u
′
)
=
0
,
h
(
u
′
(
1
)
,
u
,
u
′
)
=
0
,
where $$f:[0,1]\times \mathbb {R}^{n}\times \mathbb {R}^{n}\rightarrow \mathbb {R}^{n}$$
f
:
[
0
,
1
]
×
R
n
×
R
n
→
R
n
, $$g,h:\mathbb {R}^{n}\times C([0,1],\mathbb {R}^{n})\times C([0,1],\mathbb {R}^{n})\rightarrow \mathbb {R}^{n}$$
g
,
h
:
R
n
×
C
(
[
0
,
1
]
,
R
n
)
×
C
(
[
0
,
1
]
,
R
n
)
→
R
n
are continuous, $$\phi :\prod _{i=1}^{n}(-a_i,a_i) \rightarrow \mathbb {R}^{n}$$
ϕ
:
∏
i
=
1
n
(
-
a
i
,
a
i
)
→
R
n
, $$0<a_i\le +\infty $$
0
<
a
i
≤
+
∞
, $$\phi (s)=\left( \phi _1(s_1),\dots ,\phi _n(s_n)\right) $$
ϕ
(
s
)
=
ϕ
1
(
s
1
)
,
⋯
,
ϕ
n
(
s
n
)
and $$\phi _i:(-a_i,a_i)\rightarrow \mathbb {R}$$
ϕ
i
:
(
-
a
i
,
a
i
)
→
R
is a one dimensional regular or singular homeomorphism. Our proofs are based on the concept of the lower and upper solutions.