a "whitelist" is a list of acceptable values which you can compare an unknown value to, in order to be sure it is valid.

whitelist-1.php

<?php

// basic concept:

$whitelist = [
  'foo',
  'bar'
];

if (in_array($unknown_value, $whitelist, true)) {
  /* it's all good */
} else {
  /* nope */
}

whitelist-2.php

<?php

// a concrete example

$color = filter_input(INPUT_GET, 'color');

# Whitelist
$available_colors = ['red', 'orange', 'purple', 'brown'];

# Validation
if (in_array($color, $available_colors, true)) {
  echo "Yes, {$color} is available";
} else {
  echo 'That color is not available. Please select an available color: ', implode(', ', $available_colors);
}