How to Implement a Custom str_concat Function in a PHP Extension

Introduction

The PHP string handling functions are powerful, and their implementations are written in C within the Zend engine. This article shows how to process strings inside a PHP extension by adding a custom str_concat function.

Basic PHP Implementation

<?php
function str_concat($prefix, $string) {
    $len = strlen($prefix);
    $substr = substr($string, 0, $len);
    if ($substr != $prefix) {
        return $prefix . " " . $string;
    } else {
        return $string;
    }
}

echo str_concat("hello", "word");
echo "
";

echo str_concat("hello", "hello bo56.com");
echo "
";
?>

This function concatenates the prefix and the original string only when the original string does not already start with the given prefix.

PHP Extension Implementation

PHP_FUNCTION(str_concat)
{
    zend_string *prefix, *subject, *result;
    zval *string;
    if (zend_parse_parameters(ZEND_NUM_ARGS(), "Sz", &prefix, &string) == FAILURE) {
        return;
    }
    subject = zval_get_string(string);
    if (zend_binary_strncmp(ZSTR_VAL(prefix), ZSTR_LEN(prefix), ZSTR_VAL(subject), ZSTR_LEN(subject), ZSTR_LEN(prefix)) == 0) {
        RETURN_STR(subject);
    }
    result = strpprintf(0, "%s %s", ZSTR_VAL(prefix), ZSTR_VAL(subject));
    RETURN_STR(result);
}

The extension registers a new function str_concat. It parses a zend_string prefix and a zval string, checks whether the subject already starts with the prefix using zend_binary_strncmp, and either returns the original string or concatenates the prefix and subject.

zend_string Structure

struct _zend_string {
    zend_refcounted_h gc;   /* GC information */
    zend_ulong h;           /* hash value */
    size_t len;             /* string length */
    char val[1];            /* start of the string data */
};

zend_string

is a new structure introduced in PHP 7 to represent immutable strings efficiently.

Zend Macros

#define ZSTR_VAL(zstr)  (zstr)->val
#define ZSTR_LEN(zstr)  (zstr)->len
#define ZSTR_H(zstr)    (zstr)->h
#define ZSTR_HASH(zstr) zend_string_hash_val(zstr)

These macros provide direct access to the members of zend_string. They are defined in Zend/zend_string.h. Additional string‑related functions such as zval_get_string and zend_binary_strncmp are declared in Zend/zend_operators.h. For more macros, refer to Zend/zend_API.h.