php, create 1 array from another 2 : <p>I have two arrays like this:</p>

<pre><code>$a1 = ['A','B'];
$a2 = ['1','2'];
</code></pre>

<p>I need to have this new one (with the _ between the values):</p>

<pre><code>$a3 = ['A_1','B_2'];
</code></pre>

<p>I know isn't difficult, but I'm pretty stucked, using <code>array_combine</code> and stuff.</p>

<p>Any help? thanks</p>
