Fatal error: Cannot unset string offsets error?

I believe that you have the syntax for the foreach wrong...it should be $key => $value where you have $key => $arr2. So when you have $arr2[$key] you are looking for element $key in the nested array $arr2. $arr2 is referenced by $key, which is either a string (for an associative array) or an integer (for a non-associative array). $arr2 could also be referenced by $arr[$key].

http://php.net/manual/en/control-structures.foreach.php

You don't need the unset, you are overriding the outer parameters with the value of the inner array as opposed to the whole array.

$a1 = array("ivr");
$a2 = array("ivr2");

$a3 = array($a1, $a2);

foreach($a3 as $key => $value){
    $a3[$key] = $a3[$key][0];
    //unset($arr2[$key][0]);
};

var_dump($a3);

I think you are confused about how foreach works.

foreach($array as $key => $value)
{
  echo $key;
  echo $value;
}

will display the key and value for each key/value pair in an array.

This happens when you try to unset a string value - In below case you access the first element in the array which is a string and the try to unset it which causes this error

$a=array("hello", "there");
unset($a[0][0]);

This causes:

Fatal error:  Cannot unset string offsets in ... on line ...

I had this error in a slightly different situation which might prove useful.

unset($search['param']['complete'])

This threw the same error when $search['param'] was still a string instead of an array.