I wish to provide a client a chance to by hand download a backup of his database. I'm coding the website using PHP and MySQL. So following the admin user has drenched inside will be a link within the menu to download b .sql file towards the local computer. How do i make this happen with PHP?

This could get untidy attempting to support a database from PHP, you'd be better of letting MySql handle this, and the easiest method to tell mysql to get this done is by using a spend script:

#!/bin/bash
# Shell script to backup MySql database
# To backup Nysql databases file to /backup dir and later pick up by your
# script. You can skip few databases from backup too.
# For more info please see (Installation info):
# http://www.cyberciti.biz/nixcraft/vivek/blogger/2005/01/mysql-backup-script.html
# Last updated: Aug - 2005
# --------------------------------------------------------------------
# This is a free shell script under GNU GPL version 2.0 or above
# Copyright (C) 2004, 2005 nixCraft project
# Feedback/comment/suggestions : http://cyberciti.biz/fb/
# -------------------------------------------------------------------------
# This script is part of nixCraft shell script collection (NSSC)
# Visit http://bash.cyberciti.biz/ for more information.
# -------------------------------------------------------------------------

MyUSER="SET-MYSQL-USER-NAME"     # USERNAME
MyPASS="SET-PASSWORD"       # PASSWORD
MyHOST="localhost"          # Hostname

# Linux bin paths, change this if it can not be autodetected via which command
MYSQL="$(which mysql)"
MYSQLDUMP="$(which mysqldump)"
CHOWN="$(which chown)"
CHMOD="$(which chmod)"
GZIP="$(which gzip)"

# Backup Dest directory, change this if you have someother location
DEST="/backup"

# Main directory where backup will be stored
MBD="$DEST/mysql"

# Get hostname
HOST="$(hostname)"

# Get data in dd-mm-yyyy format
NOW="$(date +"%d-%m-%Y")"

# File to store current backup file
FILE=""
# Store list of databases
DBS=""

# DO NOT BACKUP these databases
IGGY="test"

[ ! -d $MBD ] && mkdir -p $MBD || :

# Only root can access it!
$CHOWN 0.0 -R $DEST
$CHMOD 0600 $DEST

# Get all database list first
DBS="$($MYSQL -u $MyUSER -h $MyHOST -p$MyPASS -Bse 'show databases')"

for db in $DBS
do
    skipdb=-1
    if [ "$IGGY" != "" ];
    then
    for i in $IGGY
    do
        [ "$db" == "$i" ] && skipdb=1 || :
    done
    fi

    if [ "$skipdb" == "-1" ] ; then
    FILE="$MBD/$db.$HOST.$NOW.gz"
    # do all inone job in pipe,
    # connect to mysql using mysqldump for select mysql database
    # and pipe it out to gz file in backup dir :)
        $MYSQLDUMP -u $MyUSER -h $MyHOST -p$MyPASS $db | $GZIP -9 > $FILE
    fi
done

and you tell php to inform the operating-system to complete this script and wait for this to become done, normally, this is completed with exec or system for example:

exec('/path/to/backup/script.sh');

You'll be able to simply send the file produced through the script towards the browser for your user.

I discovered this script has always labored for me personally on a number of hosts