How to reduce the image size without losing quality in PHP

You can resize and then use imagejpeg()

Don't pass 100 as the quality for imagejpeg() - anything over 90 is generally overkill and just gets you a bigger JPEG. For a thumbnail, try 75 and work downwards until the quality/size tradeoff is acceptable.

imagejpeg($tn, $save, 75);

If you are looking to reduce the size using coding itself, you can follow this code in php.

<?php 
function compress($source, $destination, $quality) {

    $info = getimagesize($source);

    if ($info['mime'] == 'image/jpeg') 
        $image = imagecreatefromjpeg($source);

    elseif ($info['mime'] == 'image/gif') 
        $image = imagecreatefromgif($source);

    elseif ($info['mime'] == 'image/png') 
        $image = imagecreatefrompng($source);

    imagejpeg($image, $destination, $quality);

    return $destination;
}

$source_img = 'source.jpg';
$destination_img = 'destination .jpg';

$d = compress($source_img, $destination_img, 90);
?>

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$d = compress($source_img, $destination_img, 90);

This is just a php function that passes the source image ( i.e., $source_img ), destination image ( $destination_img ) and quality for the image that will take to compress ( i.e., 90 ).

$info = getimagesize($source);

The getimagesize() function is used to find the size of any given image file and return the dimensions along with the file type.

I'd go for jpeg. Read this post regarding image size reduction and after deciding on the technique, use ImageMagick

Hope this helps