Return single column from a multi-dimensional array

array_map is a call back function, where you can play with the passed array. this should work.

$str = implode(',', array_map(function($el){ return $el['tag_id']; }, $arr));

Quite simple:

$input = array(
  array(
    'tag_name' => 'google'
  ),
  array(
    'tag_name' => 'technology'
  )
);

echo implode(', ', array_map(function ($entry) {
  return $entry['tag_name'];
}, $input));

http://3v4l.org/ltBZ0

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and new in php v5.5.0, array_column:

echo implode(', ', array_column($input, 'tag_name'));

join(',', array_map(function (array $tag) { return $tag['tag_name']; }, $array))

Although this question is related to string conversion, I stumbled upon this while wanting an easy way to write arrays to my log files. If you just want the info, and don't care about the exact cleanliness of a string you might consider:

json_encode($array)