c中的strlen函数字符''来判断字符串是否结束，那php呢？

typedef struct _zval_struct zval;typedef union _zvalue_value {    long lval;                 /* long value */    double dval;               /* double value */    struct {                   /* string type */        char *val;        int len;    } str;    HashTable *ht;             /* hash table value */    zend_object_value obj;} zvalue_value; struct _zval_struct {    /* Variable information */    zvalue_value value;        /* value */    zend_uint refcount__gc;    zend_uchar type;           /* active type */    zend_uchar is_ref__gc;};