php - Merge two tables and replace them

I want to merge two arrays and replace the text with strtr function.

I was using this before

$text = "cat cow";
$array = array(
"cat" => "dog",
"cow" => "bull"
);
$output = strtr($text, $array);

this returned dog bull...

Now I have two arrays like this...

$a = array("cat", "dog");
$b = array("dog", "bull");

Both the arrays will have values to replace

Now, how do I combine them and replace? I tried $array = $a + $b and array_combine, but they didn't work...

Please Help...

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I think two arrays must be

$a = array("cat", "cow");
$b = array("dog", "bull");

And you can use

$c = array_combine($a, $b);
$output = strtr($text, $c);