PHP splitting arrays into groups based on equal values

Question

I have an Array containing arrays with 2 values, the first one is the Number of the Author the second is his Affiliation.

Array ( 
    [0] => Array ( 
            [0] => 2 
            [1] => Department of General Chemistry
        ) 
    [1] => Array ( 
            [0] => 3 
            [1] => Institute of Silicate Materials
        ) 
    [2] => Array ( 
            [0] => 4 
            [1] => Department of General Chemistry
        ) 
    [3] => Array ( 
            [0] => 5 
            [1] => Department of General Chemistry
        ) 
    [4] => Array ( 
            [0] => 6 
            [1] => Institute of Silicate Materials
        ) 
)

How can I group the Authors if the Affiliation is the same? I need the output to be something like:

3,6 Institute of Silicate Materials
2,4,5 Department of General Chemistry

Answer 1

foreach ($array as $key => $value) {
 $return[$value[1]][] = $value[0];
}

foreach ($return as $key => $value) {
  echo implode(',', $value)." ".$key;
}

Answer 2

<?php
//the array
$data = array(array(2,'Department of General Chemistry'),array(3,'Institute of Silicate Materials'),array(4,'Department of General Chemistry'),array(5,'Department of General Chemistry'),array(6,'Institute of Silicate Materials'));

//a new array to store the data    
$newData = array();

//loop over each value in the data
foreach($data as $d){
    //check if a key exists under the new data for the common value (affiliation)
    if(!isset($newData[$d[1]])){
        //doesn't exist, group under the common value (affiliation)
        $newData[$d[1]] = array(array(),$d[1]);
    }
    //add the author under it's affiliation
    $newData[$d[1]][0][] = $d[0];
}

//get the values from the new data, this resets the keys
$newData = array_values($newData);

//display the data
echo '<pre>'.print_r($newData,1).'</pre>';

example

results in:

Array
(
    [0] => Array
        (
            [0] => Array
                (
                    [0] => 2
                    [1] => 4
                    [2] => 5
                )

            [1] => Department of General Chemistry
        )

    [1] => Array
        (
            [0] => Array
                (
                    [0] => 3
                    [1] => 6
                )

            [1] => Institute of Silicate Materials
        )

)

Answer 3

Try this code , it should solve your problem:

<?php
$element = array ( 
    array ( 
            2 
            ,'Department of General Chemistry'
        ) ,
    array ( 
            3, 
            'Institute of Silicate Materials'
        ), 
    array ( 
            4 
            , 'Department of General Chemistry'
        ) ,
    array ( 
            5 
            , 'Department of General Chemistry'
        ) ,
    array ( 
            6 
            , 'Institute of Silicate Materials'
        ) 
);

$res = array();

for($i=0; $i< count($element); $i++){
    $res[$element[$i][1]][] =  $element[$i][0];
}

echo '<pre>';
    var_dump($res);
echo '</pre>';

foreach ($res as $key => $value){
    echo $key .' : '. implode(',',$value). '<br>';
}
?>