Insert a value or key/value pair after a specific key in an array. If key doesn't exist, value is appended to the end of the array.

array-insert-after.php

<?php

/**
 * Insert a value or key/value pair after a specific key in an array.  If key doesn't exist, value is appended
 * to the end of the array.
 *
 * @param array $array
 * @param string $key
 * @param array $new
 *
 * @return array
 */
function array_insert_after( array $array, $key, array $new ) {
	$keys = array_keys( $array );
	$index = array_search( $key, $keys );
	$pos = false === $index ? count( $array ) : $index + 1;

	return array_merge( array_slice( $array, 0, $pos ), $new, array_slice( $array, $pos ) );
}

function push_at_to_associative_array($array, $key, $new ){
        $keys  = array_keys( $array );
		$index = array_search( $key, $keys, true  );
        $pos   = false === $index ? count( $array ) : $index + 1;
        
        $array = array_slice($array, 0, $pos, true) + $new + array_slice($array, $pos, count($array) - 1, true);
        return $array;
    }

Use Case

$your_updated_orignal_parent_array = push_at_to_associative_array($your_orignal_parent_array, $orignal_parent_array_key, $new_child_single_or_multidimentional_array );

Thanks