Erreur Notice: Uninitialized string offset

$_FILES['attfile']['name'][$g]

$machaine = 'test';
echo $machaine[0]; // affiche t
echo $machaine[1]; // affiche e
echo $machaine[2]; // affiche s
echo $machaine[3]; // affiche t
echo $machaine[4]; // erreur Uninitialized string offset: 4 -> cet offset n'existe pas

$attachments = array();
	for( $g = '0'; $g < $attfilescount; $g++ ) {
		//echo "error [" . $_FILES['attfile']['error'][$g] . "]";
	    if ( $_FILES['attfile']['error'][$g] == '4' && $_FILES['attfile']['name'][$g] == '' ){
            // no file
		}
		elseif ($_FILES['attfile']['error'][$g] == 1 || $_FILES['attfile']['size'][$g] > ($max_size * 1024)){
			// UPLOAD_ERR_INI_SIZE
			$a_errors[] = '<div class="artforms-error">Le fichier dépasse la taille maximum autorisée<br />(max. '.$max_size.' KBytes)<br />fichier : '.$_FILES['attfile']['name'][$g].'</div>';
		}
		elseif ($_FILES['attfile']['size'][$g] == 0){
			$a_errors[] = '<div class="artforms-errorwsize">Le fichier est vide<br />( '.$max_size.' KBytes )<br />fichier: '.$_FILES['attfile']['name'][$g].'</div>';
		}
		elseif (!afMimeTypeIsInArray($_FILES['attfile']['type'][$g], $mimetypen_array)){
			$chktypealert = '<div class="artforms-errorwtype">';
			$chktypealert .= 'Type de fichier incorrect' .'<br />*.'.afGetFileExt( $_FILES['attfile']['name'][$g] ).' ('.$_FILES['attfile']['type'][$g].')<br /><br />fichier : '.$_FILES['attfile']['name'][$g].'<br />';
			//$chktypealert .= '<br />'.JText::_( 'ARTF_ONLYFILESALLOWED' ).'<br /><a href="javascript:void(0);" onClick="javascript:if(document.getElementById(\'afallowedfilesspoiler'.$g.'\').style.display != \'\') { document.getElementById(\'afallowedfilesspoiler'.$g.'\').style.display = \'\';this.innerText = \'\'; this.value = \'Hide\'; } else { document.getElementById(\'afallowedfilesspoiler'.$g.'\').style.display = \'none\'; this.innerText = \'\'; this.value = \'Show\'; };">'.JText::_( 'ARTF_CLICKTOSHOW' ).'</a><div><div id="afallowedfilesspoiler'.$g.'" style="display: none;"><ul>';
			foreach ($mtypes_key_array as $key){
				$chktypealert .= "\n<li>".$mtypes_allowed[$key]['ext']."</li>";
			}
			$a_errors[] = $chktypealert;
		}
		else {
			if ($_FILES['attfile']['name'][$g] != '' ){
				$attachmentsname = str_replace(' ','-',$_FILES['attfile']['name'][$g]);
				$attachments[] = $artuplpfad.$attachmentsname;
				$attachmentstodb[] = $attachmentsname;
				move_uploaded_file($_FILES['attfile']['tmp_name'][$g],$artuplpfad.$attachmentsname);
			}
		}
	}

<input type="file" name="attfile[]" ......

if ($_FILES['attfile'][$g]['name']

if ($_FILES['attfile']['name'][$g] != '')