Simple Excel XML spreadsheet generator that can convert your 2D array to a valid XML that you can open with Excel. It's based on https://en.wikipedia.org/wiki/Microsoft_Office_XML_formats - but adds a few perks like text width calculation.

class.SimpleSheet.php

<?php

class SimpleSheet {

    private $data = [];
    private $worksheetName = "Export";
    private $widthMultiplier = 6;   //  how many points for one character; gives rough approximations
    private $cellLimiter = " ";     //  add extra character after each cell value (overflow limiter)
    private $rowLimiter = " ";      //  add extra cell at the end of each row (overflow limiter)

    public function __construct         ($a=null) {if($a) $this->loadFromArray($a);}
    public function loadFromArray       ($a) {foreach($a as $row) $this->data[]=$row;}
    public function setTitle            ($name) {$this->worksheetName = $name;}
    public function saveAs              ($filename) {file_put_contents($filename,$this->toString());}

    private function toString() {

        $C = '<Cell><Data ss:Type="String">%s%s</Data></Cell>'."\n";
        $R = '<Row>'."\n".'%s</Row>'."\n";
        $M = '<Column ss:Width="%s"/>'."\n";
        $T = '

            <?xml version="1.0" encoding="utf-8" ?>
            <?mso-application progid="Excel.Sheet"?>
            <Workbook xmlns="urn:schemas-microsoft-com:office:spreadsheet"
            xmlns:o="urn:schemas-microsoft-com:office:office"
            xmlns:x="urn:schemas-microsoft-com:office:excel"
            xmlns:ss="urn:schemas-microsoft-com:office:spreadsheet"
            xmlns:html="http://www.w3.org/TR/REC-html40">
                <Styles>
                    <Style ss:ID="Default">
                    <Alignment ss:Indent="1"/>
                    </Style>
                </Styles>
                <Worksheet ss:Name="%s">
                    <Table>
                        %s
                        %s
                    </Table>
                </Worksheet>
            </Workbook>

        ';
        $e = error_reporting();error_reporting(E_ALL - E_NOTICE); // notices are NOT errors.
        $m = $this->widthMultiplier;
        $l = $this->cellLimiter;
        $z = $this->rowLimiter;
        $N = $this->worksheetName;
        $D = $this->data;
        $z = $z?sprintf($C,"",$z):"";
        $cw=[];foreach($D  as $r) {foreach($r as $n=>$s) $cw[$n]=max($cw[$n],$this->nWidth($s));}
        $cp="";foreach($cw as $x) {$x=($x+2)*$m; if($x>300) $x=300; $cp.=sprintf($M,$x)."\n";}
        $tx="";foreach($D  as $r) {$rt="";foreach($r as $c) {$rt.=sprintf($C,$c,$l)."\n";} $tx.=sprintf($R,"$rt$z");}
        $text = sprintf($T,$N,$cp,$tx);$text = trim(preg_replace('/\s*\n\s*/',"\n",$text));
        error_reporting($e);
        return $text;

    }

    private function nWidth($str) { // how many "n"-s wide is your text; good estimation for calibri-alike fonts

        $trickySigns = "l ,:;fJ-t()[]{}!sxzGHNOUmwW";
        $similarChar = "iijjjtttttttttttcccVVVVVMMM";

        $str = strtr($str,$trickySigns,$similarChar);
        $cch = count_chars($str,1);
        $text = strlen($str)*168            // count all characters as average (letter "n")
             + $cch[ord("i")]*(  70 -168)  // compensate by differences
             + $cch[ord("j")]*(  84 -168)  // half "n"
             + $cch[ord("t")]*( 105 -168)
             + $cch[ord("r")]*( 120 -168)
             + $cch[ord("x")]*( 140 -168)  // almost the same as "n"
             + $cch[ord("V")]*( 210 -168)  // these are wider than "n"
             + $cch[ord("M")]*( 280 -168)  // very wide
        ;
        return (int)($text/168);


    }

}