$a = array('a','b','c'
);
print_r(array_flip($a));//输出为：
Array
([a] => 0[b] => 1[c] => 2
)//需要注意的是： array_flip(): Can only flip STRING and INTEGER values

//没有指定索引
$a = array('a','b',
);
$b = array('a','b',
);
$arr = array_merge($a,$b);
//输出：
Array
([0] => a[1] => b[2] => a[3] => b
)
//=======================
//显式指定数字键名
$a = array(3=>'a',4=>'b',
);
$b = array(1=>'a',2=>'b',
);
$arr = array_merge($a,$b);
//输出：
Array
([0] => a[1] => b[2] => a[3] => b
)
//=======================
//显式指定数字键名和无索引混合
$a = array(3=>'a','b',
);
$b = array('a',2=>'b',
);
$arr = array_merge($a,$b);
//输出：
Array
([0] => a[1] => b[2] => a[3] => b
)
//=======================
//显式指定非数字索引1
$a = array('a'=>1,'b',
);
$b = array('a','b',
);
$arr = array_merge($a,$b);
//输出：
Array
([a] => 1[0] => b[1] => a[2] => b
)
//=======================
//显式指定非数字索引2
$a = array('a'=>1,'b'=>2,
);
$b = array('a','b'=>22,
);
$arr = array_merge($a,$b);
//输出：
Array
([a] => 1[b] => 22[0] => a
)

//没有指定索引
$a = array('a','b',
);
$b = array('c','d','e'
);
$arr = $a + $b;
//输出：
Array
([0] => a[1] => b[2] => e
)
//=======================
//指定数字索引
$a = array(1=>'a',2=>'b',
);
$b = array(4=>'c',7=>'d'
);
$arr = $a + $b;
//输出：
Array
([1] => a[2] => b[4] => c[7] => d
)
//=======================
//指定索引，存在重复
$a = array('a'=>11,'b'=>22,
);
$b = array('c'=>33,'d'=>44,'a'=>55
);
$arr = $a + $b;
//输出：
Array
([a] => 11[b] => 22[c] => 33[d] => 44
)
//=======================
//混合指定索引和无索引1
$a = array('a',2=>'b',
);
$b = array('c','d',10=>'a',4=>22
);
$arr = $a + $b;
//输出：
Array
([0] => a[2] => b[1] => d[10] => a[4] => 22
)
//=======================
//混合指定索引和无索引2
$a = array(2=>'b','a',
);
$b = array('c','d',10=>'a',4=>22
);
$arr = $a + $b;
//输出：
Array
([2] => b[3] => a[0] => c[1] => d[10] => a[4] => 22
)

$a = array('a','b','c'
);
$b = array('a1','b1','c'
);
$arr = array_flip($a) + array_flip($b);
print_r(array_keys($arr));
//输出：
Array
([0] => a[1] => b[2] => c[3] => a1[4] => b1
)