[JusTGo]

untested:
bind key +sprint

PHP Code:

#include <amxmodx>
#include <hamsandwich>
#include <fun>

#define PLUGIN "Sprint"
#define VERSION "1.0"
#define AUTHOR "JustGo"

#define SPRINT_SPEED 300.0

#define MAX_PLAYERS 32
#define Ham_Player_ResetMaxSpeed Ham_Item_PreFrame

new bool:is_sprinting[MAX_PLAYERS+1]

public plugin_init()
{
    register_plugin(PLUGIN, VERSION, AUTHOR)
    register_clcmd("+sprint", "sprint_handler")
    register_clcmd("-sprint", "sprint_handler")
    RegisterHam(Ham_Player_ResetMaxSpeed,"player","ham_player_resetmaxspeed",1);
}

public sprint_handler(id, level, cid)
{
    
    if(!is_user_connected(id))
        return PLUGIN_HANDLED
    
    static cmd[2]
    read_argv(0, cmd, 1)
    
    switch(cmd[0])
    {
        case '+': is_sprinting[id] = true
        case '-': 
        {
            is_sprinting[id] = false
            ExecuteHam(Ham_Player_ResetMaxSpeed, id);
        }
    }
    return PLUGIN_HANDLED
}

// movement speed is changed
public ham_player_resetmaxspeed(id)
{
    if(is_sprinting[id])
    {
        set_user_maxspeed(id, SPRINT_SPEED);
    }
    return HAM_IGNORED;
}