To send an SMS from PHP through a GSM modem, open the modem’s AT-command serial port, select SMS text mode with AT+CMGF=1 if the device supports it, then use AT+CMGS to submit the recipient and message. The important part is to wait for and parse each modem response: writing bytes successfully does not prove that the modem accepted the message or that the recipient received it.
What you need before sending an SMS
PHP runs on the host computer; the modem receives commands over an exposed serial interface, often presented as a USB serial port. A device marketed as a GSM modem may expose several interfaces, and not all of them accept AT commands. Check the modem’s documentation and identify the correct port for your operating system.
- A cellular modem with SMS AT-command support, working SIM/account, and radio compatibility with your carrier and country.
- A suitable connection: USB serial, or a UART connection with compatible voltage levels and wiring.
- A PHP-side serial-port implementation that works with your operating system and installed PHP runtime. There is no single universally suitable PHP serial API established for every platform.
- The modem manual’s serial settings, including baud rate, flow control, line endings, timeouts, and command syntax.
The 3GPP SMS command family is standardized, but modem firmware, interface setup, and supported options still vary. Consult the target device’s manual alongside the ETSI TS 27.005 standard.
Choose SMS text mode or PDU mode
| Mode | Command | What it means |
|---|---|---|
| Text mode | AT+CMGF=1 |
Usually the simpler choice for a basic send workflow, if the modem supports it. Character handling and message limits remain device-dependent. |
| PDU mode | AT+CMGF=0 |
Requires the application to construct and encode the SMS protocol data unit. Use it when the device or application needs PDU-level control, and follow the modem documentation. |
The command selects a format; it does not guarantee that a particular modem supports every encoding or message type in that format. Verify behavior on the actual device.
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Send an SMS with the AT command sequence
- Connect to the AT port. Discover the modem’s command interface using the operating system and device documentation. Open it through a serial implementation compatible with your PHP runtime.
- Probe the modem. Send
ATusing the required line termination and wait for a response, commonlyOK. If there is no response, resolve port and serial settings before continuing. - Select text mode if appropriate. Send
AT+CMGF=1and read the complete response. Continue only if the modem accepts the command. - Start the send operation. Send
AT+CMGS="+15551234567", replacing the example with the destination number in the format expected by your modem and carrier. Wait for the modem’s message-entry prompt, commonly>. - Submit the message body. Only after the prompt, write the message text, then send Ctrl+Z (ASCII 26) to tell the modem to submit it. Do not send Ctrl+Z as ordinary printable text.
- Read the final result. Wait for the modem’s completion response. A message reference followed by
OKcommonly indicates acceptance by the modem; an error response means the send attempt failed. Use the modem’s supported status-report features if you need delivery information.
Conceptually, the interaction is:
AT
→ OK
AT+CMGF=1
→ OK
AT+CMGS="+15551234567"
→ >
Hello from PHP<Ctrl+Z> → +CMGS: <reference> ... OKThis is an illustration of the command exchange, not a drop-in PHP program: serial-port APIs differ across operating systems and PHP installations, and the exact responses and terminators depend on the modem. MikroTik’s RouterOS documentation likewise identifies AT+CMGS and AT+CMGF for sending, but its implementation-specific limitations should not be applied to other modems: RouterOS SMS documentation.
Read replies instead of assuming success
Build the PHP serial layer as a sequence of writes and response reads. A successful host-side write only means data was handed to the interface; the modem’s reply determines whether it accepted each command. Read until a complete response or a carefully chosen timeout, and distinguish the prompt from final results such as OK, ERROR, or extended SMS errors.
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Do not treat a modem reference or OK as proof that a handset received the SMS. Network acceptance, delivery reports, and error detail depend on modem capabilities and configuration. For more than sending, TS 27.005 also defines commands including +CMGL (list), +CMGR (read), +CMGD (delete), +CPMS (storage), +CNMI (new-message indications), and +CSCA (service-centre address).
Troubleshoot common failures
| Symptom | What to check |
|---|---|
No response to AT |
Confirm modem power, cable, selected interface, serial settings, and whether another application has the port open. Some devices expose multiple USB interfaces, with only one acting as the AT port. |
| Echoed or garbled characters | Check baud rate, flow control, line termination, and echo behavior against the modem manual. |
No prompt after AT+CMGS |
Read any error preceding the prompt. Verify SMS mode, destination syntax, SIM/account status, and whether the firmware supports the command. |
| Modem accepts the command but recipient gets nothing | Inspect the final modem result; check network registration, signal, service plan, number format, service-centre configuration where applicable, and carrier/network SMS support. Acceptance is not delivery confirmation. |
| Unicode or long message behaves unexpectedly | Check the modem’s character encoding, text-mode limits, and segmentation behavior. Test the exact message and encoding path; do not assume all modems handle concatenated or Unicode messages the same way. |
| Intermittent or corrupted commands | Serialize access to the modem port. Avoid having multiple processes issue commands simultaneously; one operation can otherwise consume another’s response or interrupt its command sequence. |
Choose a modem interface that fits the host
A USB modem with a documented SMS AT-command port is often the most direct setup for a computer, provided its drivers, radio bands, and carrier support fit the intended location. A module exposing UART can also work, but may require an adapter and correct voltage, wiring, and port configuration. Neither connection type is universally preferable: choose based on documented AT support and compatibility with the host and network.
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