Let $p$ be a prime number. Find all polynomials $P$ with integer coefficients with the following properties:
$(a)$ $P(x)>x$ for all positive integers $x$.
$(b)$ The sequence defined by $p_0:=p$, $p_{n+1}:=P(p_n)$ for all positive integers $n$, satisfies the property that for all positive integers $m$ there exists some $l\geq 0$ such that $m\mid p_l$. 
1. **Define the sequence and the polynomial properties:**
   Let \( p \) be a prime number. We need to find all polynomials \( P \) with integer coefficients such that:
   - \( P(x) > x \) for all positive integers \( x \).
   - The sequence defined by \( p_0 := p \) and \( p_{n+1} := P(p_n) \) for all positive integers \( n \) satisfies the property that for all positive integers \( m \), there exists some \( l \geq 0 \) such that \( m \mid p_l \).

2. **Analyze the sequence \( p_n \):**
   Let \( m_n = p_{n+1} - p_n = P(p_n) - p_n \). We know that \( m_n > 0 \) since \( P(x) > x \) for all positive integers \( x \).

3. **Case 1: Infinite \( n \) such that \( m_n \mid p_l \) and \( l \leq n \):**
   - If there are infinitely many such \( n \) where \( m_n \mid p_l \) and \( l \leq n \), then \( p_{n+1} - p_n \leq p_l \leq p_n \). This implies \( p_{n+1} \leq 2p_n \) for infinitely many \( n \).
   - Therefore, \( P(p_n) \leq 2p_n \) for infinitely many \( n \). This suggests that \( P(x) \) could be of the form \( P(x) = x + c \) for some constant \( c \) or \( P(x) = 2x \).

4. **Subcase 1.1: \( P(x) = x + c \):**
   - For \( P(x) = x + c \), we have \( p_{n+1} = p_n + c \). This implies \( p_n = p + nc \).
   - For \( m = c \), there should exist \( p_l = p + lc \) such that \( c \mid p_l \). This implies \( c \mid p \). Since \( p \) is a prime number, \( c \) must be either 1 or \( p \).
   - If \( c = 1 \), then \( P(x) = x + 1 \).
   - If \( c = p \), then \( P(x) = x + p \).

5. **Subcase 1.2: \( P(x) = 2x \):**
   - If \( P(x) = 2x \), then \( p_n = 2^n p \).
   - For a prime \( q \neq 2 \) and \( q \neq p \), we need \( q \mid p_l \) for some \( l \). However, \( p_l = 2^l p \), and since \( q \neq 2 \) and \( q \neq p \), this leads to a contradiction. Therefore, \( P(x) = 2x \) is not a valid solution.

6. **Case 2: For some \( n_0 \), if \( n > n_0 \), then \( m_n \mid p_l \) implies \( l > n \):**
   - From \( m_n = p_{n+1} - p_n \), it follows that \( m_n \mid p_{n+k+1} - p_{n+k} \) for every \( k \geq 0 \).
   - Since \( m_n \mid p_l \) and \( l > n \), we have \( m_n \mid p_{l-1} \), \( m_n \mid p_{l-2} \), ..., \( m_n \mid p_n \), leading to a contradiction.

Therefore, the only valid solutions are \( P(x) = x + 1 \) and \( P(x) = x + p \).

The final answer is \( \boxed{ P(x) = x + 1 } \) and \( P(x) = x + p \).