PHP 是如何做垃圾回收的

包含 php 5 与 php7 的变量实现和垃圾回收的对比

变量的实现

PHP 的变量是弱类型的，可以表示整数、浮点数、字符串等类型。PHP 的变量是使用结构体 zval 表示的

PHP 5.* zval 和 zend_value 结构

struct _zval_struct { // 结构体
    zvalue_value value;
    zend_uint refcount__gc;
    zend_uchar type;
    zend_uchar is_ref__gc;
}typedef union _zvalue_value { // 联合体
    long lval;
    double dval;
    struct {
        char *val;
        int len;
    } str; // 字符串
    HashTable *ht; // 数组
    zend_object_value obj; // 对象
    zend_ast *ast;
} zvalue_value;

PHP 7.0 zval 和 zend_value 结构

struct _zval_struct {
union {
zend_long         lval;             /* long value */
double            dval;             /* double value */
zend_refcounted  *counted;
zend_string      *str;
zend_array       *arr;
zend_object      *obj;
zend_resource    *res;
zend_reference   *ref;
zend_ast_ref     *ast;
zval             *zv;
void             *ptr;
zend_class_entry *ce;
zend_function    *func;
struct {
uint32_t w1;
uint32_t w2;
} ww;
} value;
    union {
        struct {
            ZEND_ENDIAN_LOHI_4(
                zend_uchar    type,         /* active type */
                zend_uchar    type_flags,
                zend_uchar    const_flags,
                zend_uchar    reserved)     /* call info for EX(This) */
        } v;
        uint32_t type_info;
    } u1;
    union {
        uint32_t     var_flags;
        uint32_t     next;                 /* hash collision chain */
        uint32_t     cache_slot;           /* literal cache slot */
        uint32_t     lineno;               /* line number (for ast nodes) */
        uint32_t     num_args;             /* arguments number for EX(This) */
        uint32_t     fe_pos;               /* foreach position */
        uint32_t     fe_iter_idx;          /* foreach iterator index */
    } u2;
};