<?php
$standard_image_2 = 'This is the variable variable 2';
$standard_image_4 = 'This is the variable variable 4';

$steps = array(
 0=>array(
  'number'=>2
 ),
 1=>array(
  'number'=>4
 )
);

foreach ($steps as $step) {
  echo ${'standard_image_'.$step['number']}; 
}

?>

This is the variable variable 2This is the variable variable 4