Here's a bigger list of data types.  I got this by creating every type I could and calling fetch_fields():

<?php
$mysql_data_type_hash = array(
    1=>'tinyint',
    2=>'smallint',
    3=>'int',
    4=>'float',
    5=>'double',
    7=>'timestamp',
    8=>'bigint',
    9=>'mediumint',
    10=>'date',
    11=>'time',
    12=>'datetime',
    13=>'year',
    16=>'bit',
    //252 is currently mapped to all text and blob types (MySQL 5.0.51a)
    253=>'varchar',
    254=>'char',
    246=>'decimal'
);
?>

This may be obvious, but the constants for the field types are already defined in PHP, and can be found in the documentation at: http://php.net/manual/en/mysqli.constants.php

According to
http://www.redferni.uklinux.net/mysql/MySQL-Protocol.html

Data type values are:

DECIMAL           0       ENUM           247
TINY              1       SET            248
SHORT             2       TINY_BLOB      249
LONG              3       MEDIUM_BLOB    250
FLOAT             4       LONG_BLOB      251
DOUBLE            5       BLOB           252
NULL              6       VAR_STRING     253
TIMESTAMP         7       STRING         254
LONGLONG          8       GEOMETRY       255
INT24             9
DATE             10
TIME             11
DATETIME         12
YEAR             13
NEWDATE          14

Note that this is not tested and does not contain
all the values noted by deluxmozart

According to
dev.mysql.com/sources/doxygen/mysql-5.1/mysql__com_8h-source.html
the flag bits are:
 
 NOT_NULL_FLAG          1         /* Field can't be NULL */
 PRI_KEY_FLAG           2         /* Field is part of a primary key */
 UNIQUE_KEY_FLAG        4         /* Field is part of a unique key */
 MULTIPLE_KEY_FLAG      8         /* Field is part of a key */
 BLOB_FLAG             16         /* Field is a blob */
 UNSIGNED_FLAG         32         /* Field is unsigned */
 ZEROFILL_FLAG         64         /* Field is zerofill */
 BINARY_FLAG          128         /* Field is binary   */
 ENUM_FLAG            256         /* field is an enum */
 AUTO_INCREMENT_FLAG  512         /* field is a autoincrement field */
 TIMESTAMP_FLAG      1024         /* Field is a timestamp */

Here are two methods for converting the 'type' and 'flags' attributes to text for debugging purposes. They both use the predefined MYSQLI_ constants to generate the text.

<?php

public static function h_type2txt($type_id)
{
    static $types;

    if (!isset($types))
    {
        $types = array();
        $constants = get_defined_constants(true);
        foreach ($constants['mysqli'] as $c => $n) if (preg_match('/^MYSQLI_TYPE_(.*)/', $c, $m)) $types[$n] = $m[1];
    }

    return array_key_exists($type_id, $types)? $types[$type_id] : NULL;
}

public static function h_flags2txt($flags_num)
{
    static $flags;

    if (!isset($flags))
    {
        $flags = array();
        $constants = get_defined_constants(true);
        foreach ($constants['mysqli'] as $c => $n) if (preg_match('/MYSQLI_(.*)_FLAG$/', $c, $m)) if (!array_key_exists($n, $flags)) $flags[$n] = $m[1];
    }

    $result = array();
    foreach ($flags as $n => $t) if ($flags_num & $n) $result[] = $t;
    return implode(' ', $result);
}

?>

Warning!
This function is not only more memory consuming than expected but the consumption also depends on the size of the result set. So if you realy only want to get your field_names you might want to append " LIMIT 1" or use mysqli->unbuffered_query() to save yourself from a memory bloat.

Beware of the fact that the ->def property NEVER gets filled with the correct default field value, so it's totally USELESS.

This happens NOT for a bug in php (so don't go filling in a bug report), but happens BY DESIGN, since the MySQL C API call doesn't fill in this value, unless you call the mysql_list_fields() function, which Php doesn't.

See here for reference.
http://dev.mysql.com/doc/refman/5.0/en/c-api-datatypes.html

Also, be aware that if you're using a query which contains subqueries, the primary key/autoincrement flags do NOT get passed along, even if the field you're looking at is the primary autoincrement key of the master table:

SELECT * from (SELECT id from areas) AS subareas

and you'll find primary key and autoinc flags off on id field, even if id was the primary autoinc key of areas table.

This also is by design, i think, since it's supposed that if we're using a subquery then the primary key/autoinc stuff might have no sense at all, since in the result set we can compose fileds from many different tables.

Hoping this is useful, bye!

You can find all available data types here:
https://php.net/manual/ru/mysqli.constants.php
(search "MYSQLI_TYPE_" in your browser)

JSON type is 245 if anyone is wondering.

note that
"SELECT <timestamp-field>, ..."
will return the field with type 7 (timestamp) but with content like "2010-07-14 14:35:08". the point being it is a string.

"SELECT <timestamp-field> + 0, ..."
returns a type 5 (double) but while a number, is not seconds since epoch, but a number in MySQL's "YYYYMMDDHHMMSS" format, in this example:
20100714143508

(PHP 5.2.12)

Here are some Numbers of Datatypes. I searched for it but i didn't find a list, where the datatypes of the numbers are listed.

so first I can give this:

    3    -    Int
    10    -    Date
    246    -    Decimal
    252    -     text
    253    -    VarChar
    254    -    Boolean

I wrote a simple class to get info about fields.
Try it!

<?php
            class MysqlFieldsInfo implements Iterator
            {
                
                private $result;
                private $position;
                private $row;
                
                
                function __construct($result){
                    $this->result = $result;
                    $this->position = 0;
                    $this->rewind();    // W $results wewnętrzny wskaźnik może być przesunięty więc powracamy do początku
                }
                
                public function current(){
                    return $this->row;
                }
                
                public function next(){
                    $this->position++;
                    $this->row = $this->result->fetch_field();    
                }
                
                public function valid(){
                    return (boolean) $this->row;
                }
                
                public function key(){
                    return $this->position;
                }
                
                public function rewind(){
                    $this->position = 0;
                    $this->result->field_seek(0);
                    $this->next();
                }
                
                //    This function show data in table
                public function export(){
                    
                    echo '<table id="db_table_info">';
                    echo '<tr>
                                                <th>Name</th>
                                                <th>Orgname</th>
                                                <th>Table</th>
                                                <th>Orgtable</th>
                                                <th>Def</th>
                                                 <th>Max_length</th>
                                                 <th>Length</th>
                                                 <th>Charsetnr</th>
                                                 <th>Flags</th>
                                                 <th>Type</th>
                                                  <th>Decimals</th>
                                          </tr>';
                    while($this->valid()){
                        echo '<tr>';
                        printf("\n\t<td>%s</td>\n", $this->current()->name);
                        printf("\t<td>%s</td>\n", $this->current()->orgname);
                        printf("\t<td>%s</td>\n", $this->current()->orgtable);
                        printf("\t<td>%s</td>\n", $this->current()->def);
                        printf("\t<td>%s</td>\n", $this->current()->max_length);
                        printf("\t<td>%s</td>\n", $this->current()->length);
                        printf("\t<td>%s</td>\n", $this->current()->charsetnr);
                        printf("\t<td>%s</td>\n", $this->current()->flags);
                        printf("\t<td>%s</td>\n", $this->current()->type);
                        printf("\t<td>%s</td>\n", $this->current()->decimals);

                        echo '</tr>';        
            
                        $this->next();
                    }
                    
                    echo '</table>';
                }
            }
?>