How do I dynamically create the variable name in a PHP loop?

Ok so i have this php foreach loop

<?php foreach ($step_count->rows as $step) { ?>

and $step will be the step numbers 1, 2, 3, 4, 5 up to the total steps

within the loop i a need to set the value of the images within the loop to standard_image_1 or whatever step there is...so for example

<input value="<?php echo {$standard_image_"$step['number']"}; ?>" />

so basically i need the variable $standard_image_1 and so on depending on the steps but i dont know the correct syntax to do this