Oder ... nach Rolfs Idee

[ursus contionabundo]

Ich dachte, er will zuerst Großbuchstaben. Du meinst, er will:

<?php
$final_array[0]['score']='bar';
$final_array[1]['score']='Foo';
$final_array[2]['score']='Bar';
$final_array[3]['score']='foo';

usort( $final_array, function( $a, $b ) {
	echo 
	    '"' . $a['score'] . '" <=> "' . $b['score'] . '" : '
	    . ( $a['score'] <=> $b['score'] )  
		  . ' => ' 
		  . ( -1 * ( $a['score'] <=> $b['score'] ) ) 
		  . PHP_EOL;

    return -1 * ( $a['score'] <=> $b['score'] );
} );

print_r( $final_array );

mit diesem Ergebnis?

"bar" <=> "Foo" : 1 => -1
"Foo" <=> "Bar" : 1 => -1
"Bar" <=> "foo" : -1 => 1
"Foo" <=> "foo" : -1 => 1
"bar" <=> "foo" : -1 => 1
Array
(
    [0] => Array
        (
            [score] => foo
        )

    [1] => Array
        (
            [score] => bar
        )

    [2] => Array
        (
            [score] => Foo
        )

    [3] => Array
        (
            [score] => Bar
        )

)