Generating randomized pairs of elements in php

I want to generate random pairs of elements and have this code:

<? 
$myarray1=array('1','2','3'); 
$myarray2=array('1a','2a','3a'); 

echo '<p>'.$myarray1[array_rand($myarray1)].'</p>'; 
echo '<p>'.$myarray2[array_rand($myarray2)].'</p>'; 
?>

but I get this as a result:

<p>3</p>
<p>2a</p>

or

<p>1</p>
<p>3a</p>

or

<p>2</p>
<p>1a</p>

etc.

and I want to only get certain pairs:

<p>2</p>
<p>2a</p>

or

<p>1</p>
<p>1a</p>

or

<p>3</p>
<p>3a</p>

etc.

how do I achieve that?