so this works with phps mail function, sending sms to mobile. you have to know the carrier being used in order for this to work...essentially every phone number is also an email address within its carrier....so i use verizon (i know, they sux), when you text me 757-757-7575, its really sending an email to my phoneNumber@carrier (7577577575@vtext.com).
this is very basic, in working demo, i've even hardcoded my carrier and number as the input values, although that may not be ideal for you.
essentially here's the mail function:
$message = wordwrap( $_REQUEST['smsMessage'], 70 );
$to = $_REQUEST['phoneNumber'] . '@' . $_REQUEST['carrier'];
$result = @mail( $to, '', $message );
and the form markup
<form action="" method="post">
<ul>
<li>
<label for="phoneNumber">Phone Number</label>
<input type="text" name="phoneNumber" id="phoneNumber" placeholder="7577597204" value="7577597204" /></li>
<li>
<label for="carrier">Carrier</label>
<input type="text" name="carrier" id="carrier" placeholder="vtext.com" value="vtext.com" /></li>
<li>
<label for="smsMessage">Message</label>
<textarea name="smsMessage" id="smsMessage" placeholder="Text Message Goes Here Yo!..."></textarea>
</li>
<li>
<input type="submit" name="sendMessage" id="sendMessage" value="Send Message" /></li>
</ul>
</form>
i think i've done quite a wretched job explaining this, so i'm sorry if there's any confusion. here's working demo: http://dev.bowdenweb.com//php/sms/working-sms-demo2.html
and i also saved it as a text file so you can see the php
http://dev.bowdenweb.com//php/sms/working-sms-demo2.txt
sorry i don't have a repo on github for this yet...i should probably do that today.
edit: here's a mailto demo using my sms email...it goes through the user's email, which is actually quite awkward. not sure why i wanted to try it...but it was useful for me to see the pre-populated email being created before i send them out.
http://jsfiddle.net/jalbertbowdenii/7jFee/