We support Bulk SMS. The API remains the same as sending a single SMS, as given here: How to send an SMS. 

Send Bulk SMS With Static Content (refer to this)

To send bulk SMS, specify an array of "To" numbers instead of a single number. The API contract remains the same.

Sample PHP bulk SMS API code sample is present at Sample Bulk Code

Send Bulk SMS With Dynamic Content (refer to this)

This API will send SMSes to different numbers with different message content for each number.