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PHP FFI Internals

bookPHP zval to C Type Conversion

PHP FFI (Foreign Function Interface) bridges PHP and native C code by translating PHP variablesβ€”known internally as zvalsβ€”to their closest C equivalents. This conversion process is critical for correct data exchange and function calls between PHP and C. Most simple scalar types are mapped automatically: PHP int values are converted to C int or long types, float to C float or double, and string to C char * (with a null terminator added). However, not all conversions are seamless. For example, PHP bool is mapped to C int (with true as 1 and false as 0). When using FFI, PHP will attempt to convert the value you pass to the closest compatible C type, but for more complex or ambiguous types, explicit casting may be necessary. For instance, when passing a PHP string to a function expecting a pointer to a struct, you must first create a suitable FFI object. Automatic conversion is limited to scalars and simple pointers; anything more complex requires careful handling.

main.php

main.php

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<?php
// Define a simple C function: int add_ints(int a, int b)
$ffi = FFI::cdef("
    int add_ints(int a, int b);
    double add_doubles(double a, double b);
    int accept_string(const char* str);
", "
    int add_ints(int a, int b) {
        return a + b;
    }
    double add_doubles(double a, double b) {
        return a + b;
    }
    int accept_string(const char* str) {
        return (str != NULL) ? (int)strlen(str) : -1;
    }
");

// Passing int values (automatic conversion)
echo $ffi->add_ints(2, 3), PHP_EOL; // 5

// Passing float values to a function expecting int (automatic conversion, but with truncation)
echo $ffi->add_ints(2.7, 3.9), PHP_EOL; // 5 (floats are truncated to ints)

// Passing string to a function expecting int (conversion fails)
try {
    echo $ffi->add_ints("hello", 3), PHP_EOL;
} catch (FFI\Exception $e) {
    echo "Error: ", $e->getMessage(), PHP_EOL;
}

// Passing float values to a C double function (automatic conversion)
echo $ffi->add_doubles(2.5, 3.5), PHP_EOL; // 6.0

// Passing int values to a C double function (automatic conversion)
echo $ffi->add_doubles(2, 3), PHP_EOL; // 5.0

// Passing PHP string to a function expecting const char* (automatic conversion)
echo $ffi->accept_string("hello"), PHP_EOL; // 5

// Passing PHP int to a function expecting const char* (conversion fails)
try {
    echo $ffi->accept_string(123), PHP_EOL;
} catch (FFI\Exception $e) {
    echo "Error: ", $e->getMessage(), PHP_EOL;
}
?>

A common mistake when working with PHP FFI is attempting to pass incompatible PHP types to C functions. For example, passing a PHP array to a parameter expecting an int will not workβ€”PHP cannot automatically convert arrays to numeric C types, and this will cause a runtime error or, worse, undefined behavior. Similarly, passing an object or resource to a C pointer type without proper conversion will lead to errors. Always ensure that the PHP value's type matches the expected C type, or perform explicit casting or conversion using FFI's facilities before passing the value.

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Thanks for your feedback!

SectionΒ 2. ChapterΒ 2

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bookPHP zval to C Type Conversion

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PHP FFI (Foreign Function Interface) bridges PHP and native C code by translating PHP variablesβ€”known internally as zvalsβ€”to their closest C equivalents. This conversion process is critical for correct data exchange and function calls between PHP and C. Most simple scalar types are mapped automatically: PHP int values are converted to C int or long types, float to C float or double, and string to C char * (with a null terminator added). However, not all conversions are seamless. For example, PHP bool is mapped to C int (with true as 1 and false as 0). When using FFI, PHP will attempt to convert the value you pass to the closest compatible C type, but for more complex or ambiguous types, explicit casting may be necessary. For instance, when passing a PHP string to a function expecting a pointer to a struct, you must first create a suitable FFI object. Automatic conversion is limited to scalars and simple pointers; anything more complex requires careful handling.

main.php

main.php

copy

              
1234567891011121314151617181920212223242526272829303132333435363738394041424344454647

            
<?php
// Define a simple C function: int add_ints(int a, int b)
$ffi = FFI::cdef("
    int add_ints(int a, int b);
    double add_doubles(double a, double b);
    int accept_string(const char* str);
", "
    int add_ints(int a, int b) {
        return a + b;
    }
    double add_doubles(double a, double b) {
        return a + b;
    }
    int accept_string(const char* str) {
        return (str != NULL) ? (int)strlen(str) : -1;
    }
");

// Passing int values (automatic conversion)
echo $ffi->add_ints(2, 3), PHP_EOL; // 5

// Passing float values to a function expecting int (automatic conversion, but with truncation)
echo $ffi->add_ints(2.7, 3.9), PHP_EOL; // 5 (floats are truncated to ints)

// Passing string to a function expecting int (conversion fails)
try {
    echo $ffi->add_ints("hello", 3), PHP_EOL;
} catch (FFI\Exception $e) {
    echo "Error: ", $e->getMessage(), PHP_EOL;
}

// Passing float values to a C double function (automatic conversion)
echo $ffi->add_doubles(2.5, 3.5), PHP_EOL; // 6.0

// Passing int values to a C double function (automatic conversion)
echo $ffi->add_doubles(2, 3), PHP_EOL; // 5.0

// Passing PHP string to a function expecting const char* (automatic conversion)
echo $ffi->accept_string("hello"), PHP_EOL; // 5

// Passing PHP int to a function expecting const char* (conversion fails)
try {
    echo $ffi->accept_string(123), PHP_EOL;
} catch (FFI\Exception $e) {
    echo "Error: ", $e->getMessage(), PHP_EOL;
}
?>

A common mistake when working with PHP FFI is attempting to pass incompatible PHP types to C functions. For example, passing a PHP array to a parameter expecting an int will not workβ€”PHP cannot automatically convert arrays to numeric C types, and this will cause a runtime error or, worse, undefined behavior. Similarly, passing an object or resource to a C pointer type without proper conversion will lead to errors. Always ensure that the PHP value's type matches the expected C type, or perform explicit casting or conversion using FFI's facilities before passing the value.

question mark

Which of the following statements about PHP zval to C type conversion via FFI is correct?

Select the correct answer

Everything was clear?

How can we improve it?

Thanks for your feedback!

SectionΒ 2. ChapterΒ 2
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