will only decode the data if it's base64 (or a base64 data URI), if it's not will return the same data, also used to check if it's base64 or a data URI.

decode_if_b64.php

function decode_if_b64($file_content="",$return_boolean=false) {
    
    /**
    * 
    * Base64 files handling
    * 1.   first we'll check if it's a base64 data URI
    *      and we'll strip the data uri initials
    * 
    * 2. Confirm it using regular expression
    * 
    * 3. Confirm it again with PHP base64_decode() function
    * 
    * 4. Decode it
    * 
    **/
    
    // it's not, unless proven otherwise
    $is_base64 = false;
    $is_data_uri = false;
    
    // initials
    $indi_pos1 = strpos($file_content,"data:");
    $indi_pos2 = strpos($file_content,";base64,");
    
    if(($indi_pos1 !== false) && ($indi_pos2 !== false)) {
        // it's base64 data URI.
        // strip out those initails
        $file_content_striped = substr($file_content,$indi_pos2+8);
        $is_data_uri = true;
        
        /**
        * 
        * Note how we're saving the stripped data into a new string not mutilitaing the original
        * file contents, until we have other checks returns true.
        * 
        * this userful incase of text files that has the string ;base64, in them.
        * 
        **/
        
    }
    
    $base64_preg_check = preg_match("~^([A-Za-z0-9+/]{4})*([A-Za-z0-9+/]{4}|[A-Za-z0-9+/]{3}=|[A-Za-z0-9+/]{2}==)$~",$file_content_striped);
    
    // confirm with regular expression
    if($base64_preg_check){
        $decodedBase64 = base64_decode($file_content_striped,true);
        // if decoding was done correctly with no errors
        if($decodedBase64) {
            $file_content = $decodedBase64;
            $is_base64 = true;
        }
        
        /**
        * Note: we could have relied on the base64_decode to check if it's really a base64 file.
        * but, for performance issues we'll only try to decode if the file is matched the regular expresion
        * since regular expression matching is faster than base64_decode function. 
        * 
        **/
    }
    
    if($return_boolean) $return_data = array(
        "base64"    =>  $is_base64,
        "dataURI"   =>  $is_data_uri,
        "file"      =>  $file_content
    );
    
    else $return_data = $file_content;
    
    return $return_data;
}