foo.c

// gcc -g -shared $(php-config --includes) -L$(php-config --prefix)/lib -lphp7 -o reopcode.so -fPIC reopcode.c
// sudo ./inject -p $(pgrep -n php) reopcode.so

#include <php.h>
#include <zend_API.h>
#include <zend_vm.h>

extern ZEND_API void (*zend_execute_ex)(zend_execute_data *execute_data);
static void (*orig_execute_ex)(zend_execute_data *execute_data);

const char *peek_fname = "/tmp/test.php";
const char *peek_func = "testfn";
// const char *peek_var = 'y';
uint32_t peek_lineno = 7;

static void inject_opcode(zend_op_array *op_array) {
    int iold;
    zend_op *old;
    iold = 0;
    while (iold < op_array->last) {
        old = &op_array->opcodes[iold];
        if (old->lineno == peek_lineno) {
            old->opcode = ZEND_EXIT;
            zend_vm_set_opcode_handler(old);
            break;
        }
        ++iold;
    }
}

static void my_execute_ex(zend_execute_data *execute_data) {
    zend_function *zf;
    zend_op_array *op_array;

    zf = execute_data->func;
    op_array = &zf->op_array;

    if (   0 == strcmp(peek_fname, ZSTR_VAL(op_array->filename))
        && zf->common.function_name
        && 0 == strcmp(peek_func,  ZSTR_VAL(zf->common.function_name))
    ) {
        inject_opcode(op_array);
    }
    orig_execute_ex(execute_data);
}

static void __attribute__((constructor)) loadMsg() {
    orig_execute_ex = zend_execute_ex;
    zend_execute_ex = my_execute_ex;
}