// 135 chars – supposing notices are silenced through error_reporting
function f($n,&$l){foreach(str_split($n)as$c){$b=0;foreach([9=>0,Z=>A,z=>a]as$m=>$i)$c<=$m&&$c>=$i&&$b=join(range($i,$c));$l.=$b?:$c;}}

// 147 chars – the same without notices
function f($n,&$l){foreach(str_split($n)as$c){$b=0;foreach(['9'=>'0','Z'=>'A','z'=>'a']as$m=>$i)$c<=$m&&$c>=$i&&$b=join(range($i,$c));$l.=$b?:$c;}}

// 140 chars – still no notices, but less strings
function f($n,&$l){foreach(str_split($n)as$c){$b=0;foreach(['90','ZA','za']as$m)$c<=$m[0]&&$c>=$m[1]&&$b=join(range($m[1],$c));$l.=$b?:$c;}}