How to fix Warning Illegal string offset in PHP

Magic word is: isset

Validate the entry:

if(isset($manta_option['iso_format_recent_works']) && $manta_option['iso_format_recent_works'] == 1){
    $theme_img = 'recent_works_thumbnail';
} else {
    $theme_img = 'recent_works_iso_thumbnail';
}

1.

 if(1 == @$manta_option['iso_format_recent_works']){
      $theme_img = 'recent_works_thumbnail';
 } else {
      $theme_img = 'recent_works_iso_thumbnail';
 }

2.

if(isset($manta_option['iso_format_recent_works']) && 1 == $manta_option['iso_format_recent_works']){
    $theme_img = 'recent_works_thumbnail';
} else {
    $theme_img = 'recent_works_iso_thumbnail';
}

3.

if (!empty($manta_option['iso_format_recent_works']) && $manta_option['iso_format_recent_works'] == 1){
}
else{
}

Please check that your key exists in the array or not, instead of simply trying to access it.

Replace:

$myVar = $someArray['someKey']

With something like:

if (isset($someArray['someKey'])) {
    $myVar = $someArray['someKey']
}

or something like:

if(is_array($someArray['someKey'])) {
    $theme_img = 'recent_works_iso_thumbnail';
}else {
    $theme_img = 'recent_works_iso_thumbnail';
}