Map array with initial letter in PHP - php

Question

I wish to group a word list in an array with the initial letter.
function alpha($str) {
$result[substr($str,0,1)] = $str;
return $result;
}
$a = ['abc','cde','frtg','acf'];
$b = array_map('alpha', $a);
print_r($b);
What I need:
Array
(
[a] => abc,acf
[c] => cde
[f] => frtg
)
What I get:
Array
(
[0] => Array
(
[a] => abc
)
[1] => Array
(
[c] => cde
)
[2] => Array
(
[f] => frtg
)
[3] => Array
(
[a] => acf
)
)

Answer 1

How about that :
$answer = [];
$a = ['abc','cde','frtg','acf'];
foreach($a as $word){
$key = substr($word,0,1);
if (isset($answer[$key])){
$answer[$key] .= "," . $word;
} else {
$answer[$key] = $word;
}
}

Answer 2

Just add a variable $c and loop over arrays of array using two foreach and group by alphabet...
function alpha($str) {
$result[substr($str,0,1)] = $str;
return $result;
}
$a = ['abc','cde','frtg','acf'];
$b = array_map('alpha', $a);
#print_r($b);
$c = [];
foreach ($b as $key => $values) {
foreach ($values as $key => $value) {
if(!isset($c[$key])){
$c[$key]=$value;
}else{
$c[$key].= "," . $value;
}
}
}
echo "<PRE>";
print_r($c);
Outupt:
Array
(
[a] => abc,acf
[c] => cde
[f] => frtg
)

Answer 3

The function array_map maps to the original indexes but you want new indexes and an altered array, if there are more values with the same initial character. Therefore array_map don't work for you. You could create your new array this way:
$a = ['abc','cde','frtg','acf'];
$b = Array();
$c = Array();
foreach( $a as $v )
{
// multidimensional array
$b[substr($v,0,1)][] = $v;
// comma separated string
$c[substr($v,0,1)] = (isset($c[substr($v,0,1)])) ?
$c[substr($v,0,1)].",$v" : $v;
}

Answer 4

If the first character can also be multibyte Unicode such as ° or €, mb_substr() must be used! Solution with foreach:
$result = [];
$a = ['abc','€de','frtg','acf'];
foreach($a as $word){
$key = mb_substr($word,0,1);
$result[$key] = array_key_exists($key,$result)
? ($result[$key].",".$word)
: $word
;
}
Solution with array_reduce():
$result = array_reduce($a,function($carry,$item){
$key = mb_substr($item,0,1);
$carry[$key] = array_key_exists($key,$carry) ? ($carry[$key].",".$item) : $item;
return $carry;
},[]);
var_export($result);
Output:
array (
'a' => 'abc,acf',
'€' => '€de',
'f' => 'frtg',
)

Answer 5

I think your intention is to store each word into arrays according to its first letter. In this case, the multidimensional array is the right choice.
$array = ['abc','cde','frtg','acf'];
$new_array = array();
foreach($array as $v){
$letter = substr($v,0,1);
if(!isset($new_array[$letter])) {$new_array[$letter] = array();}
array_push($new_array[$letter], $v);
}
print_r($new_array);