Gammu modules comes with gammu_sendsms() function to send SMS.

Syntax: gammu_sendsms($gammu_destinationnumber, $gammu_textdecoded, $SendingDateTime = NULL)

Examples:

1. Send simple SMS
   gammu_sendsms('082111978168', "Hello word");
2. Send simple SMS at specific date time
   gammu_sendsms('082111978168', "Hello word", '2012-10-25 12:00:00');
3. Send simple SMS with line break
   gammu_sendsms('082111978168', "Hello\nword");