Get index for multi-dimensional array - php

Question

I need to get an index of a multidimensional and output it as a variable. I know this is very simple but I am struggling with it.
For example given the following:
[1]=>
array(11) {
["field_label"]=>
string(24) "What is your first name?"
["field_name"]=>
string(6) "f_name"
["identifier"]=>
bool(true)
["options"]=>
bool(false)
["form_name"]=>
string(12) "demographics"
}
[2]=>
array(11) {
["field_label"]=>
string(23) "What is your last name?"
["field_name"]=>
string(6) "l_name"
["identifier"]=>
bool(true)
["options"]=>
bool(false)
["form_name"]=>
string(12) "demographics"
}
[3]=>
array(11) {
["field_label"]=>
string(32) "Researcher who took measurements"
["field_name"]=>
string(17) "weight_researcher"
["identifier"]=>
bool(false)
["options"]=>
bool(false)
["form_name"]=>
string(6) "weight"
}
I want to find the index for the first element that has a form_name of "weight" (#3)

Answer 1

Just use a foreach and an if inside it:
foreach($array as $key => $value) {
// ^ here resides the key of the parent array
if($value['form_name'] == 'weight') { // if form name is weight
echo $key; // echo the key
break; // then stop on first occurence
}
}

Answer 2

PHP >= 5.5.0 needed for array_column:
$key = array_search('weight', array_column($array, 'form_name'));
For multiple keys:
$keys = array_keys(array_column($array, 'form_name'), 'weight');