$orderDoc = array("ORT", "TRI", "CONT", "RMI");

$FindDoc= array("RMI0000191","ORT0000379","ORT0000391","ORT0000392","ORT0000390","CONT0000274","CONT0000275","RMI0000192","ORT0000391");

array(10) {
  [0]=>
  string(10) "RMI0000191"
  [1]=>
  string(10) "ORT0000379"
  [2]=>
  string(10) "ORT0000391"
  [3]=>
  string(10) "ORT0000392"
  [4]=>
  string(10) "ORT0000390"
  [5]=>
  string(11) "CONT0000274"
  [6]=>
  string(11) "CONT0000275"
  [7]=>
  string(10) "RMI0000192"
  [8]=>
  string(10) "ORT0000394"
  [9]=>
  string(10) "TRI0000170"
}

$FindDoc=asort($FindDoc,$orderDoc );

PHP - Sort Functions For Arrays
In this chapter, we will go through the following PHP array sort functions:

sort() - sort arrays in ascending order
rsort() - sort arrays in descending order
asort() - sort associative arrays in ascending order, according to the value
ksort() - sort associative arrays in ascending order, according to the key
arsort() - sort associative arrays in descending order, according to the value
krsort() - sort associative arrays in descending order, according to the key

array(10) {
  [0]=>
  string(10) "ORT0000379"
  [1]=>
  string(10) "ORT0000390"
  [2]=>
  string(10) "ORT0000391"
  [3]=>
  string(10) "ORT0000392"
  [4]=>
  string(10) "ORT0000394"
  [5]=>
  string(10) "TRI0000170"
  [6]=>
  string(11) "CONT0000274"
  [7]=>
  string(11) "CONT0000275"
  [8]=>
  string(10) "RMI0000191"
  [9]=>
  string(10) "RMI0000192"
}