PHP 配列関数 array_combine() 配列マージ関数

$a1=array("a","b","c","d");
$a2=array("猫","犬","馬", "牛");
print_r(array_combine($a1,$a2));

配列 ( [a] => 猫

=> 犬 [c] => 馬 [d] => 牛 )