PHP get previous array element knowing current array key

Solution with fast lookups: (if you have to do this more than once)

$keys = array_flip(array_keys($array));
$values = array_values($array);
return $values[$keys[555]-1];

array_flip(array_keys($array)); will return an array mapping keys to their position in the original array, e.g. array(420 => 0, 430 => 1, 555 => 2).

And array_values() returns an array mapping positions to values, e.g. array(0 => /* value of $array[420] */, ...).

So $values[$keys[555]-1] effectively returns the previous elements, given that the current one has key 555.

Alternative solution:

$keys = array_keys($array);
return $array[$keys[array_search(555, $keys)-1]];

You can iterate through the array in reverse and return the next iteration after finding the search value.

$found = false;
foreach(array_reverse($array, true) as $key=>$value) {
  if ($found) {
    print "$key => $value\n";
    break;
  } else if ($key == 555) {
    $found = true;
  }
}

http://ideone.com/2WqmC

I solved this issue in this way:

function getPrevKey($key, $hash = array())
{
    $keys = array_keys($hash);
    $found_index = array_search($key, $keys);
    if ($found_index === false || $found_index === 0)
        return false;
    return $keys[$found_index-1];
}

@return previous key or false if no previous key is available

Example:

$myhash = array(
    'foo' => 'foovalue',
    'goo' => 'goovalue',
    'moo' => 'moovalue',
    'zoo' => 'zoovalue'
);

echo "TEST: ". getPrevKey('zoo', $myhash); // prints moo

Just iterate over the array

$_index = null;
foreach($myarray as $index => $value)
{
    if($key == $my_index) // if($key == 550)
    {
        break;
    }
    $_index = $index;
}

echo $_index; //the prev key from 550;

An alternative solution is to get the keys of your array within an enumerated array like so:

$keys = array_keys($my_array);

as the keys array is index you can move the the previous key like so:

$required_key = (array_search(550,$keys,true) - 1);

this will fine the value of 550, and return its index within the keys, remove one to get the previous index

key we have our previous key to get the value from the original array

$value = $my_array[$required_key];

This is a simple solution for taking previous and next items, even if we are at the ends of the array.

<?php
    $current_key; // the key of the item we want to search from

    if (isset($array[$current_key+1])) 
    {

        $array_next = $array[$current_key+1]; // get the next item if there is 
    } 
    else 
    {       
        $array_next = $array[0]; // if not take the first (this means this is the end of the array)             
    }       

    if (isset($array[$current_key-1])) 
    {

        $array_prev = $array[$current_key-1]; // get the previous item if there is 
    } 
    else 
    {   
        $array_prev = $array[count($array)-1]; // if not take the last item (this means this is the beginning of the array)             
    }

?>

@Luca Borrione's solution was helpful. If you want to find both previous and next keys, you may use the following function:

function getAdjascentKey( $key, $hash = array(), $increment ) {
    $keys = array_keys( $hash );    
    $found_index = array_search( $key, $keys );
    if ( $found_index === false ) {
        return false;
    }
    return $keys[$found_index+$increment];
}

Usage:

// previous key / returns false if result found at first key
getAdjascentKey( $key, $hash, -1 );

// next key / returns false if result found at last key
getAdjascentKey( $key, $hash, +1 );