PHP 1:1 to 1:many array

Question

I have three arrays called associativeArray keyArray and valueArray. associativeArray is an array that's made up of key/value pairs, the pairs are split up and placed into keyArray and valueArray. What I want to do now is create a fourth array called newArray that uses the elements in valueArray as its keys and have the values be from their respective keyArray. But unlike the associativeArray that has 1:1 key-to-value, I want to have newArray have 1:many key-to-value while not having any repeating keys. This is the code I made for it:

foreach($keyArray as $keyElement){
    $valueElement = $associativeArray[$keyElement];
    if (!in_array($valueElement,$newArray)){
        array_push($newArray, $valueElement => array($keyElement));
    }
    else{
        array_push($newArray[$valueElement],$keyElement);
    }
}

However whenever I run it I get:

PHP Parse error:  syntax error, unexpected T_DOUBLE_ARROW

Answer 1

You don't need all those arrays. Just associativeArray is enough.

You can do it like this:

$newArray = array();
// This will loop in the array so that, in each step:
//  - $k is the current key
//  - $v is the current value
foreach($associativeArray as $k => $v) {
    // If newArray doesn't already have $v as a key
    if (!array_key_exists($v, $newArray)) {
        // Define it as a new array with only one element
        $newArray[$v] = array($k);        
    } else {
        // If it already exists, just push $k to it's elements
        $newArray[$v][] = $k; // This is the same as array_push($newArray[$v], $k)
    }
}

Answer 2

The simplest solution ever could be something like this

foreach( $keyArray as $key )
{
  // get the value from the associative array
  $value = $associativeArray[ $key ];

  // check if the key was set before into $newArray
  // if you don't check and initialize it, you'll get an error
  // when you try to add new elements
  if( !array_key_exists( $key, $newArray ) )
  {
       $newArray[ $key ] = array();
  }

  // To prevent duplicated ( like you have in your code ) 
  // you just have to check if the element is in the $newArray
  if ( !in_array( $value, $newArray[ $key ] ) )
  {
      $newArray[ $key ][] = $value; // if you wanna use array_push --> array_push( $newArray[ $key ], $value );
  }
}

But, it depends on your goal, I would rewrite the associativeArray making use of reference and casting the value in an array. At that point you don't need those 3 extra arrays but just associativeArray and one line of code

// this will cast each value of the array. Since
// the value is referenced the array will be updated.
// You don't have to check for duplicated value because it was an 
// array 'key' => 'val' so it is not possible to have multiple values 
// with the same key
foreach( $associativeArray as $key => &$val )
{
   $val = (array) $val;
}