Check if POST is the same as the variable urls / myfile.php

Let's see if I understand correctly:

You have a FORM that sends a data for POST ... and you want that FORM to tie the password input with a variable of a PHP file.

If this is how I understand you, it's simple:

Do an include of your file urls/miarchivo.php , hence the variable $ password that is in miarchivo.php is already accessible, in case there is a problem at most you can include it to become global ( global $password ) and ready is comparable.

Let's see it in code (remember, this code is simple you should not use it for security and so on):

<?php
     include("urls/miarchivo.php"); //se incluye el archivo donde se encuentra la variable $password
     $passwordDelForm = $_POST['password']; //esta es la contraseña que manda el form

     if($passwordDelForm != ""){ //se checa que no esté vacía
           if($passwordDelForm == $password){ //si empatan las variables
                echo "Contraseña correcta";
           }else{ //si no empatan
                echo "Error en la contraseña";
           }
      }else{ //si está vacía le mostramos un error
           echo "Error en la contraseña";
      }
?>

You can corroborate after the include by tests printing the variable to see if it loads you and it does not have problems of accessibility, in case this is only as mentioned

<?php
    global $password = "holamundo";
?>

I hope you serve, greetings and luck.